WBCHSE Class 12 Physics Semiconductor Electronics Notes

WBCHSE Class 12 Physics Notes

Semiconductors And Electronics Conduction Of Electricity In Solids

A solid can in general be divided Into two classes:

1. Crystalline and
2. Amorphous.

We shall concern ourselves In this chapter only with crystalline solids. A lattice in such a solid is an ordered sequence of points describing the arrangement of y atoms that form a crystal.

A unit cell is defined as the smallest part of a crystal that repeats itself regularly through translation in three dimensions to form the entire crystal. Innumerable unit cells are arranged in a regular pattern to form a piece of a crystal.

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Based on electrical conductivity, solids can be divided Into three groups:

1. Conductor
2. Insulator and
3. Semiconductor

1. Conductor:

Electricity can easily pass through the conductors. Solid conductors are mainly metals. In metals free elec¬ trons act as charge carriers; the magnitude of current through metals may be equal to or greater than 1 A. With an Increase In temperature, the resistance of metallic conductors increases. Silver (Ag), copper (Cu), aluminum (Al), iron (Pc), etc. are some examples of conductors.

2. Insulator:

The substances through which electricity cannot pass are known as insulators. Free electrons do not exist In this kind of substance. Most of the non-metals are insulators. The insulators which are widely used in electrical machines are mica, diamond, quartz, etc.

3. Semiconductor:

Semiconductors are substances that possess electrical conductivity that falls between that of metals and insulators. Conduction Conduction in semiconductors occurs through the movement of free electrons and holes. The electric current flowing through this substance

The current never exceeds a few milliamperes. Semiconductors such as silicon (Si) and germanium (Ge) exemplify this category. The distinguishing characteristic of silicon and germanium Both of them belong to the fourth group (carbon group) in the periodic table and have -1 electrons in their outermost orbit, which are crucial for the formation of covalent bonds. During the process of electrical conduction, silicon or germanium crystals utilise free electrons and holes as charge carriers, as previously explained.

As the temperature rises, their resistance diminishes. The upcoming part will provide a comprehensive analysis of this subject matter. Out of the several types of semiconductors, silicon is favoured due to its low production cost in the manufacturing of electronic devices. Nevertheless, silicon-germanium (SiGe) is employed as a substitute for silicon (Si) in high-speed networking.

There are two types of semiconductors:

1. Pure or intrinsic semiconductors and
2. Impure or extrinsic semiconductors

WBBSE Class 12 Semiconductor Electronics Notes

Pure Or Intrinsic Semiconductor

1. Valcnco electron:

5 atoms of silicon (Si) crystal. There are 4 electrons in the outermost shell of a silicon atom. Each electron forms a single covalent bond with an electron of the adjacent silicon atom. So, the four covalent bonds.

In this scenario, the effective number of electrons in the outermost shell of the central atom becomes 8. These electrons enclosed in the bonding are called valence electrons, thus Each atom in the crystal gets stability by fulfilling an octet in the outermost shell and keeps the crystal In uniform bonding.

At absolute zero temperature (i.e., 0 K), each electron remains confined to the bond. Due to the absence of any free electron or hole, conduction of electricity does not take place through the crystal, i.e., the crystal behaves as an insulator.

2. Conduction electron:

Now if the temperature of the crystal is increased, i.e., if the crystal absorbs heat, the energy of the valence electrons increases. Due to this, some valence electrons gain sufficient kinetic energy to break the covalent bond and come out from the valence shell. These electrons are known as free electrons and they act as charge carriers in the crystal. If the suitable potential difference is applied, current flows through sil¬ icon or germanium crystals due to these electrons. These charge-carrying free electrons are called conduction electrons or thermal electrons.

Note that, the conductivity of a substance is directly proportional to its concentration of free electrons, n. in the case of a good conductor, n ≈ 10²³ electrons per m³, and for an insulator, n ≈ 10⁷ electrons per m³. In the case of semiconductors, the value of n lies between these two. For example, at room temperature (i.e., at 300K), the values of n for germanium and silicon are n ≈10¹⁹ per m3 and n ≈10¹⁶ per m³ respectively.

3. The magnitude of current:

The value of electric current through pure germanium crystal is not more than a few microamperes (μA) and in the case of silicon, it is even less, only a few nanoamperes (nA). This small current is of no use for practical purposes and hence pure semiconductor has no use as an electrical element.

4. Resistance Of semiconductor:

As the temperature of a silicon or germanium crystal rises, the quantity of unbound electrons within it also rises. Consequently, the electric current likewise rises, meaning that the resistance of the crystal reduces. Conversely, the temperature of a metallic conductor directly affects its resistance, causing it to grow. The relationship between temperature and resistance in metallic conductors and semiconductors

5. Hole:

WBCHSE class 12 physics notes Hole Definition:

If any electron is released from a bond in an atom, the deficit of the electron at that position is regarded as a hole. Its effective charge is +e, although it is not a real particle.

Generation of holes:

Assuming a pure silicon crystal, the electron originally located at position A in bond 1 is relocated to place X upon the bond’s rupture. Therefore, a valence electron undergoes a transformation and becomes a conduction electron. Simultaneously, there is an electron deficiency at location A. An effective positive charge is generated at the specified place.

A pertains to its adjacent electrons. A deficiency of electrons within a bond is referred to as a hole. The charge of an electron is -e, while the effective charge of a hole is +e. In an intrinsic semiconductor, the number of holes generated is precisely equivalent to the number of electrons released, denoted as n = p, where n and p represent the concentrations of electrons and holes, respectively.

Short Notes on p-n Junctions

The motion of holes:

Apply a potential difference across the two ends of the crystal. The bond at location B in bond 2 is disrupted as a result of thermal vibration of the electron. Now, this electron will undergo directed motion as a result of the applied potential difference and eventually occupy the vacancy at position A of bond 1. Simultaneously, the aperture at point A will disappear and a fresh aperture will materialise at location B. It can be inferred that the hole at location A gets moved to location B. The electrons in a crystalline material move in the opposite direction to the holes.

It is important to note that the concept of a ‘hole’ is not an actual particle like an electron. Instead, it refers to the absence or lack of an electron. It is simply a theoretical framework created to develop a model for elucidating conduction in semiconductors. Inside. Describing electric current in terms of hole motion is often beneficial. Electrons are the primary negative charge carriers in silicon and germanium crystals, whereas holes are regarded as the primary positive charge carriers.

WBCHSE class 12 physics notes Energy Bands in Solids

We shall begin by considering a sodium atom as an example. An isolated Na atom has been. Its electronic configuration is ls²2s²2p⁶3s¹.

The nucleus of an atom creates an attractive force that forms a potential well (shown by a solid line). Electrons are then placed in this potential well by occupying various discrete energy levels with negative potential energies. Pauli’s Exclusion Principle states that each energy level can accommodate a maximum of two electrons with opposite spins.

Therefore, in the Sodium atom, two electrons occupy each of the energy levels Is and 2s, while the 2p level is divided into 3 sublevels and contains 6 electrons. The valence electron occupies the 3s orbital. This electron is the outermost electron of the sodium atom.

Within the sodium crystal lattice, numerous atoms are in close proximity to one another, resulting in a modification of the potential well’s form, as seen by the solid line. Typically, with the exception of the valence electron, the remaining electrons in the Na atom are located within their respective potential well. Therefore, these electrons are not affected by adjacent atoms.

But, in the case of valence electrons, the situation is completely different. The valence1, electrons cannot ^be accommodated within the potential well. Therefore each of the valence electrons is influenced by all the other atoms surrounding it.

• So, it is not possible to recognize the valence electrons of individual Na atoms in the 3s energy level.
• However, according to Pauli’s exclusion principle, the maximum number of electrons that can be accommodated in a definite energy level is two.
• Due to this, the 3s energy level splits into a large number of substrates Each substate contains 2 electrons. As inside a crystal, large numbers of atoms (~ 10²⁰) are packed closely in a very small space, the number of substrates is very high.
• So the variation of the potential energy of the energy levels may be assumed to be continuous. Thus these closely spaced energy levels will form an energy band at the position of 3s.
• This energy band in a solid crystal is called the valence band.
• Generally, each electron in the valence band escapes from’ its atom, but due to attraction by the rest of the ionized atoms i.e., a group of atoms in a crystal, the valence electrons cannot behave as free electrons.
• So no free electrons are available as charge carriers.
• In the crystal, if the valence electrons gain sufficient energy from an external source to overcome the potential barrier of the group of atoms, then the electrons become free.
• These electrons are called conduction electrons. Now, if a potential difference is applied at both ends of a solid sodium bar, then the conduction electrons start drifting.

For this current will be introduced in the solid. When a large number of valence electrons are transformed Into conduction electrons by acquiring enough energy, they form an energy residing In a certain discrete energy level. This energy band is j called the conduction band.

Common Questions on Semiconductor Materials

Naturally, the energy of electrons in the conduction band is more than that in the valence band. The gap between these two consecutive energy bands is known as the forbidden zone.

No electron can stay in the forbidden zone. The energy gap between these two bands is known as the forbidden energy gap. For different substances, the energy gaps (between these two bands are different, and depending on the energy gap, the electrical conductivities of different substances are also different.

Semiconductor electronics class 12 notes  Insulator:

In an insulator, the energy gap between the valence band and the conduction band is such that, electrons in the valence band can never gain sufficient energy for transition into conduction electrons As a result, no charge carriers are produced and the substance behaves as an insulator.

Conductor:

Overlap occurs between the higher region of the valence band and the lower region of the conduction band in certain compounds. The movement of electrons from the valence band to the conduction band does not require any energy.

Consequently, valence electrons have the ability to readily convert into conduction electrons. Therefore, a vast number of charge carriers are generated, causing the substance to exhibit excellent conductivity.

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Semiconductor:

The substances in which the energy gap between the valence band and conduction band is smaller than insulators behave as semiconductors.

• The energy required for the transition of valence electrons into conduction electrons is greater than that for conducting substances.
• However, the energy gap between the valence band and conduction band in these sub¬stances is not too large like that in insulators. This energy gap  (E_g) is 0.67 eV for germanium and 1.11 eV for silicon

Electrons and holes in the two bonds:

At low temperatures, the valence band of an intrinsic semiconductor remains saturated and the conduction band remains fully vacant. Now, at a higher temperature, when an electron reaches the conduction band from the valence band, a vacancy of electron is created in the valence band, i.e., a hole is generated.

Naturally, the number of electrons in the conduction band and the number of holes in the valence bond of an intrinsic semiconductor are equal

Semiconductor Electronics Class 12 Notes

Ranges of resistivity and conductivity:

The following table provides an overall knowledge of the resistivities and conductivities of the three types of materials

Comparison of Semiconductors with Conductors and Insulators

Semiconductor electronics class 12 notes 

Semiconductors And Electronics Impure Or Extrinsic Semiconductor

Important Definitions Related to Semiconductor Electronics

Impure Or Extrinsic Semiconductor Definition:

If some special kinds of impurities are mixed with an intrinsic semiconductor in a controlled manner, the conductivity of the semiconductor increases drastically. The semiconductor thus developed is known conductor. The method of mixing of impurities is called doping and the mixed impurities are called dopants.

A few milliamperes (10^–3 A) current can be passed through silicon or germanium crystals by doping with impurities. The current increases to a high value because the energy gap between the valence band and the conduction band decreases considerably due to doping. As a result, the concentration of charge carriers in the crystal increases manyfold.

Extrinsic semiconductors are of two types 

1. n-type and
2. p – type.

1. n-type semiconductors

n-type  Definition:

If pentavalent elements (like arsenic or phosphorus) are doped as Impurities In the crystal of an Intrinsic semiconductor In a controlled manner, the crystal thus formed is called an n-type semiconductor.

n-type  Structure:

A small amount of pentavalent impurity [of group V (nitrogen group) elements,] like arsenic (As) or phosphorus (P), is doped in a controlled way in pure silicon (Si) or germanium (Ge) crystals to produce this kind of semiconduc¬ tors. Each dopant atom contains 5 electrons in its outermost orbit.

n-type  Working principle: 

A silicon crystal doped with arsenic is shown. Inside the crystal, an arsenic atom finds itself surrounded by silicon atoms. It forms four covalent bonds with 4 neighboring silicon atoms. The extra electron in its outermost orbit finds no place to occupy in any bond and hence acts as a free electron or conduction electron. These electrons are known as donor impurities. If only 1 phosphorus or arsenic atom is doped per 10⁶ germanium or silicon atoms, a sufficient number of conduction electrons are released, thereby raising the electrical conductivity of the crystal by a factor of 10⁶.

The energy band of an n-type semiconductor has been shown. The dotted line indicates the energy level of the excess electrons generated by the doping of the pentavalent element. These electrons can easily be excited to the conduction band. This line is known as donor level.

n-type Discussions:

•  The n-type crystal, as a whole, is chargeless. Electrons don’t need to remain actively charged because although free, the arsenic or phosphorus atoms present inside the crystal are electrically neutral.
• The energy gap between the Fermi level and the conduction band is approximately 0.05 eV.  In n-type semiconductors, the majority carriers are electrons, and the minority carriers are holes.
• Only a single impurity atom on average is to be doped in approximately 106 atoms of the original crystal. Hence the host silicon or germanium crystal should be an absolute purc. The cost of a semiconducting crystal is almost entirely due to its purification.
• But still, the crystal is very cheap. | y| Phosphorus or arsenic atoms donate free electrons to the pure semiconducting crystal and hence they are called donor elements.

Since the negatively charged electrons act as majority charge carriers, this type of crystal is called r;-type

Class 12 Physics Semiconductor Notes

2. P-type semiconductors

P-type Definition:

If trivalent elements (like boron or aluminum) are doped as impurities in the crystal of an intrinsic semiconductor in a controlled manner, the crystal thus obtained is called a p-type semiconductor

p-type Structure:

A small amount of trivalent impurity (of group III elements,) like boron (B), and aluminum (Al). is doped in a controlled manner in pure silicon (Si) or germanium (Gi) crystals to produce this kind of semiconductor. Each dopant atom contains 3 electrons In Its outermost orbit.

P-type Working principle:

A silicon crystal doped with boron. Inside the crystal, a boron atom finds itself surrounded by four silicon atoms of which the boron atom completes 3 covalent bonds with 3 neighboring silicon atoms.

Due to the deficit of one electron in the outermost orbit of boron. the fourth bonding cannot be completed and hence a hole appears there. If a suitable potential difference is applied, the holes having effective positive charges can drift inside the crystal. The holes act as charge carriers. As a result, it Is possible to bring the electrical conductivity of the crystal up to its desired value.

The energy band of the p-type semiconductor is shown. The dotted line denotes an electron-accepting level. The electron of the valence band can easily be exited to the acceptor level. Thus carrier holes are created in the valence band.

Its majority carriers are holes and its electrical conductivity is many times greater than that of an intrinsic semiconductor.

P-type Discussions:

• The p-type crystal as a whole is electrically neutral
• The energy gap between the valence band and the fermi level is approximately 0.05 eV.
• In p-type semiconductors, the majority carriers are holes and the minority carriers are electrons.
• Only a single atom on average is to be doped in approximately 106 atoms of the original crystal
• When boron or aluminum atoms are doped in the semi¬ conducting crystal, holes are generated which can accept electrons. Thus, boron or aluminum are called acceptor elements.

Since positively charged holes act as majority charge carriers in this type of crystal, it is called p-type.

Difference between n-type and p-type Semiconductors

Difference between n-type and p-type Semiconductors:

Drift of Charge Carriers in Semiconductors

In the case of current conduction through a metal wire, the current through the wire is

I = nevA

Where e = charge of an electron, v = drift velocity of free electrons through the metallic wire n = number density or concentration of free electrons = number of free electrons in unit volume of metal, and A = area of cross-section.

Class 12 physics semiconductor notes Intrinsic semiconductors:

Consider a cylindrical block of Intrinsic semiconductor of length I and area of cross-section A. Here, electron-hole pairs act as charge carriers. Let potential difference V be applied across two ends of the semiconductor block. As a result, both electrons and holes start drifting in opposite directions. As electrons are negatively charged and holes are positively charged, so current will flow in the same direction for both the charges. Conventionally, this direction is along the drifting of holes. i.e opposite to the drifting of electrons

So, the current passing through the semiconductor
.
I = nev_eA+ pev_hA = Ae(nv_e+pv_h) ……………………………………… (1)

Here, n = number density of electrons

p = Number density of holes

v_e = Drift velocity of electrons

v_h = Drift velocity of holes

For intrinsic semiconductors; the number of thermal electrons and thermal holes as charge carriers are the same.

Therefore, n = p = n_i, where ni is the number density of electron-hole pairs in intrinsic semiconductors.

So, from the equation (1), we get

I = Aen_i(v_e+ v_h) ………………………………………… (2)

The current through unit area i.e current density for equation(1)

J = \(\frac{I}{A}\) =   e(nv_e+pv_h) ………………………………………. (3)

For intrinsic semiconductor

J = en_i(v_e + v_h)

The effective electric field in the semiconductor is E = \(\frac{V}{l}\)

So, J = \(\frac{I}{A}=\frac{1}{A} \frac{V}{R}\)

I = \(\frac{V}{R}\)

Or, J = \(\frac{1}{A} \cdot \frac{V}{\rho \frac{l}{A}}=\frac{1}{\rho} \frac{V}{l}\)

= \(\frac{1}{\rho} \frac{V}{l}=\frac{1}{\rho} E\)

= σE

Here , R = \(\frac{\rho l}{A}\) = Resistance of the block of semiconductor

ρ = Restivity

And \(\sigma=\frac{1}{\rho}\) = conductivity

hence σ E= e(nv_e+pv_h) from equation (3)

Or, σ =  \(e\left(n \frac{v_e}{E}+p \frac{v_h}{E}\right)=e\left(n \mu_e+p \mu_h\right)\) ………………………………… (4)

Here  μ_e = \(\frac{v_e}{E}\) = Mobility of electrons

And = \(\mu_h=\frac{v_h}{E}\)  = Mobility of holes

This is the equation of conductivity of the semiconductor crystal

In the case of intrinsic semiconductors

σ = \(e n_i\left(\mu_e+\mu_h\right)\) ……………………………..(5)

It should be borne in mind that in an intrinsic semiconductor, charge carrier electrons reside outside atomic bonds, whereas charge carrier holes remain inside atomic bonds. Therefore the holes cannot move as easily as electrons.

So, it can be said that the effective mass of each hole is larger than that of each electron. Hence, at the time of current flow through the semiconductor, drift velocity and mobility of electrons are larger than those of holes i.e.,

ν_e>ν_h and μ_e >μ_h

It is important to note that each equation depends on the temperature T of the semiconductor. So, the number densities, drift velocities, and mobilities of electrons and holes are changed with temperature change.

Extrinsic semiconductor:

We have seen how in an n-type semiconductor, electrons act as majority carriers, and in a p-type semiconductor, holes in the host crystal act as the majority carriers.  Hence, in an extrinsic semiconductor, the number densities of electrons and holes are not the same i.e., n ± p

In an n-type semiconductor, as the number of majority charge carriers increases by doping, the holes get annihilated due to recombination with newly generated electrons.  Hence, with an increase in electron concentration (n), the hole concentration (p) gradually decreases. On the other hand, for p-type semiconductors, n decreases with the increase of p. In either case, the condition of equilibrium is

np =  n²_i ……………………….. (6)

The equation (6) is known as the law of mass action.

Law of mass action:

Under thermal equilibrium, the product of free electron concentration (n) and freehold concentration (p) is constant. This constant is equal to the square of carrier concentration (n) in intrinsic semiconductor

Incidentally in SI, the charge of an electron or hole is 1.6 × 10^-19 C

Unit number density is m^-3, a unit of drift velocity is m.s^-1 V unit of mobility is m² V^-1.s^-1 and the unit of conductivity is mho m^-1 i.e., Ʊ  .m^-1 or, S .m^-1

Semiconductor devices class 12 notes 

Semiconductors And Electronics Impure Or Extrinsic Semiconductor Numerical Examples

Example 1.  An intrinsic semiconductor has 5 × 10²⁸ atoms and a carrier concentration of 1.5 × 10¹⁶ m^-3. If it is doped by a pentavalent impurity in the ratio 1: 10^-6, then calculate the number density of holes as charge carriers  
Solution:

On doping by pentavalent Impurity, the n-type semiconductor is formed. Here we neglect the number density of the thermal electrons lu respect to (ho number density of electrons as majority carriers. Nonce the number density of electrons in n-type semiconductors,

n = \(\frac{5 \times 10^{28}}{10^5}\)

= 1.5 × 10¹⁶ m^-3

Given, n_i = 1.5 × 10¹⁶ m^-3 holes,

p = \(\frac{n_i^2}{n}=\frac{\left(1.5 \times 10^{16}\right)^2}{5 \times 10^{22}}\)

= 4.5 × 10⁹ m^-3

Practice Questions on Semiconductor Electronics

Example 2. A semiconductor has equal electron and hole concentrations of 6 ×10⁸ m^-3. On doping with a certain Impurity, the electron concentration of the semiconductor Increases to 8×10¹² m^-3.

1. What type of semiconductor is obtained from doing?
2. Calculate the new hole concentration of the semiconductor.

Solution:

Here, n = p = n_i = 6 ×10⁸ m^-3 i.e., in its initial state, it is an intrinsic semiconductor.

As the concentration of electrons as majority carriers increases on doping, so n-type semiconductor is formed in n-type semiconductor, and the electron concentration

In n-type semiconductor, the electron concentration

n = 8 × 10¹² m^-3

Her, np= n²_i

Therefore, the new hole concentration

p = \(\frac{n_i^2}{n}=\frac{\left(6 \times 10^8\right)^2}{8 \times 10^{12}}\)

= 4.5 × 10⁴m^-3

Example 3. A semiconductor has an electron concentration of 0.45× 10¹²m^–3 and a hole concentration of 5 × 10²⁰m^–3. Calculate the conductivity of the material of this semiconductor. Given, Electron mobility = 0.135 m².V^-1.s^-1
Solution:

Here, n = 0.45 × 10¹²m^–3

p = 5 × 10²⁰m^–3

μ = 0.135 m².V^-1.s^-1

μ = 0.048 m².V^-1.s^-1

So, the conductivity of the material of this semiconductor

σ = e(nμ_e+ pμ_h)

= 1.6 × 10^-19(0.45× 10¹²)× 0.135 +(5×10²⁰) ×0.048

= 3.84 mho.m^-1

Semiconductors And Electronics p- n junction

p-n junction Definition

By the opposite kind of doping, if one part of a semiconducting crystal is made n-type p-type, then that crystal is called a p-n junction. end me ot

A p-n junction is shown- There is a jnncaoc in between the p-type and n-type parts. Remember that, by jor-ing two different p-type and n-type crystals, a p-n junction is not formed because in that case, the crystals would not be joined uniformly and so the junction would not act properly.

To construct a p-n junction diode, a single crystal is doped with p -Type in one half and n type in the other half The junction thus developed is not so perfect though impossible to make the junction thin up to the order of lOÿm so far. Here we will take the junction as an ideal one to avoid complexity. It means we consider a perfect junction between p and. n -parts

Circuit symbol

The symbol by which a p-n junction a denoted in an electrical circuit. The base of the triangle indicates p-end and the line drawn at the vertex parallel to the base indicates n-end.

Depletion region or depletion layer

As soon as the p-n junction is formed, electrons and holes start to diffuse through the junction.

First, we take a look at the condition of the junction beer. diffusion starts. During that time, the free electrons in the n-pan move disorderly, whereas the donor ions remain fixed in their positions. As the free electrons remain confined within the n-part, the net charge of this part is zero. On the other hand, the free holes in the p -part move disorderly, whereas the acceptor ions remain fixed. Just like the n -part, the net charge of the p – part is also zero.

Due to the lack of free electrons in the p -part, the free electrons scan to move from n to the p -part- Similarly, due to the lack of free boles in n pan, the free holes start to move from p to n -pan, Le., diffusion starts. In this tray, the electrons just entering the p-part neutralize the holes of the acceptor ions near the junction.

This makes the acceptor ions negative in charge, i.e., the net charge in the p pan becomes negative. Similarly, the holes just entering the n-pan neutralize the electrons of the donor ions near the junction. This makes the donor ions positive in charge, i.e., the net charge in n-pan becomes positive.

Because of diffusion, the amount of positive and negative charges in n -part, and p -part respectively increases rapidly. At a certain moment, the amount of these charges becomes so high that no electron or hole can cross the junction anymore. In other words, net dif¬fusion in this state comes to zero.

In this condition up to a certain distance from either side of the plane no free charge depletion region. electron whole P S n depletion region it does exist. The region on either side of the junction plane containing no free charge is known as the depletion region (or depletion layer).

Inevitably, the depletion region contains negative and positive ions in the p -part and the n -part respectively. Hence due to higher potential in n -type and lower potential in p -type, a potential barrier develops at the p-n junction plane. Neither the majority carriers nor the electron and hole can overcome this barrier.

However, the movement of minority carriers continues even after the development of the depletion region. Since the depletion layer in the n -n-region is at a positive potential, the minority carrier electrons of the depletion layer in the p -p-region get attracted toward that direction. Again, the minority carrier holes of the depletion layer in the n -n-region get attracted toward the depletion layer in the -region having a negative potential.

Semiconductor Devices Class 12 Notes

Application of forward and reverse bias to the p-n junction

Biassing refers to the process of establishing a connection between electrical components. An electron behaves similarly to electronic components such as a diode or transistor when an external source of electricity, such as a battery, is applied. Prior to the application of any external voltage across a p-n junction, it has been previously discussed that there is no biassing or influence present.

The N-end acquires a positive potential, while the P-end acquires a negative potential. The phenomenon being described here is the application of a reverse bias to the junction, sometimes referred to as the natural reverse bias. Consequently, a depletion layer is spontaneously created around the location of the n and p areas. The working principle of a p-n junction is determined by the transition of the depletion layer when an external bias is applied, namely the actual external voltages applied to the junction.

1. Application of reverse bias:

Reverse bias is applied p-n junction by connecting the n-end of the p-n junction with the positive terminal of the external source and the p-end with the negative terminal.

If reverse bias is applied using an external battery B to a p-n junction, the thickness of the depletion region increases. The majority carriers cannot cross the junction and hence no current is obtained in the external circuit But due to the motion of minority carriers, a small current is obtained whose value in the case of germanium is approximately 10^-6 A’ and in base of silicon, it is only about 10^-6 A. The current is called. the reverse saturation current of a diode. in most cases, this current is neglected

2. Application of forward bias:

To apply forward bias to a p-n junction, its p -end is connected with the positive terminal of the external source of electricity, and n -ends with the negative terminal. A part of the forward bias applied using an external battery B, is used to decrease the value of the potential barrier. To do so applied voltage is to be increased. Very soon, the voltage thus applied reaches a particular value, when the depletion region vanishes.

If the forward bias is increased more the holes present in the p -region and electrons in the n -region can cross the junction easily. This is due to the applied positive potential at the end and negative potential at the n -end which help the holes and electrons, respectively to pass through the junction. As a result, a current flows through the external circuit According to the conventional rule, the direction of current flow in the external circuit is just opposite to the direction of flow of electrons, i.e., the direction along which the holes flow.

3. Semiconductor diode:

If forward bias is applied to a p-n junction current flows through it; but when a reverse bias is applied current flow is negligible. So, the p-n junction acts as a valve, i.e., the current through it is unidirectional. p-n junction is also called a semiconductor diode. It can be used as a rectifier just like a vacuum diode, although their properties are not identical

Characteristics of p-n junction diode: The variation of current with potential difference applied to a p-n junction diode in its forward and reverse biased condition, is shown in

It is known as I-V characteristics or simply the characteristic curve of a p-n junction.

Some properties of the characteristic curve:

• Due to the presence of minority carriers, a reverse saturation current exists in reverse bias.
• To neutralize the reverse saturation current, a minimum forward biasing is essential
• With the increase in the potential difference in forward bias, the current increases rapidly (AB part in that curve).

The characteristic curve of the p-n junction is not linear, i.e., V and I are not proportional to each other. Hence it is a non-ohmic electrical component. Since ΔI is the change of current due to a change of potential difference  ΔV, the ratio ΔV/ΔI is called the dynamic resistance of the junction. The value of the dynamic resistance Rp is different in the different portions of the characteristic curve.

Semiconductor devices class 12 notes p-n junction rectifier

The arrangements convert an alternating waveform into unidirectional wav.-f.mn an alternating current into the unidirectional current. It called. rectifier. A p-n junction diode is used for the rectification of alternating current

Half-wave rectification

Tire required a circuit diagram for half-wave reeducation, the input as well as the output forms. For the positive half-cycle of alternating current, the p-n junction gets forward biased, and for the biased. So, only for the positive half cycle of the input alternating voltage output voltage and current are obtained which is unidirectional. Since only one half-cycle of the input wave can be rectified by this arrangement, it is called half-wave rectification.

Each wave-crest in the DC output is called a ripple. In a half¬ wave rectifier, the number of wave crests in the alternating input becomes equal to the number of ripples in the DC output. Hence, if the frequency of alternating Input is 50 Hz, the frequency of the ripples will also be 50 Hz.

Full-wave rectification:

The full-wave rectification and the input-output waveforms shown A full wave can be rectified by using two p-n junctions.

For one half-cycle of the alternating current, the diode D₁ gets forward bia&d but tire diode D₂ gets reverse biased. As a result, current flows only through the diode D₁, in this case. For the text half-cycle, the diode D₂ gets forward biased but the diode D₁ gets reverse biased. As a result, current flows only through tile diode D₂. Note that, for both the half-cycles of a complete cycle, tire current through the load resistance is unidirectional.

Since, both the half-cycles of the input wave can be rectified by this arrangement, so it is called full-wave rectification. In this case, the tire number of ripples becomes double the number of wave-crests of tire alternating input. Hence, if the frequency of tire alternating input is 50 Hz, the frequency of the ripples will be 100 Hz.

Advantage of silicon over germanium for use as rectifiers: Due to tire presence’ of minority carriers, a very small current passes through the junction in the reverse bias. Hence, a p-n junction is not completely free from error as a rectifier. The value of this reverse current is approximately 10-6 A for germanium and only 10~9 A for silicon. This reverse current can easily be neglected for silicon. Hence, silicon is more useful than germanium as a rectifier

Semiconductors And Electronics p- n junction Numerical Examples

Example 1. The potential barrier of a p-n junction diode is 0.4 V. If the thickness of the depletion region is 4.0 × 10^-7 m, what will be the electric field intensity in this region? An electron from the n-region moves towards the p-n junction with a velocity of 6 × 10s m s^-1. What will be the velocity of that electron with which it enters the p -p-region?

Electric field intensity, E = \(\frac{V}{d}\)

Here, V = value ofpotential barrier = 0.4 V

And d = thickness of depletion region = 4 × 10^-7 m.

∴ E = \(\frac{0.4}{4 \times 10^{-7}}\)

E = 10⁶ V.m^-1

Let an electron enter to the depletion region from the n -n-region with velocity v₁ and come out from the depletion region with velocity v₂. Due to this, the increase in potential energy is eV. According to the principle of conservation of energy

½mv²₁ = eV+ ½mv²₂

Or,  ½ (9.2 ×10^-31)× (6×10⁵)²

= 1.6 ×10^-19×  0.4 + ½ ×(9.1×10^-31)

Or, 1.64 ×10^-19 = 0.64 ×10^-19  +4.55×10^-31. v²₂

v²₂ = \(\frac{1 \times 10^{-19}}{4.55 \times 10^{-31}}\)

= 22 × 10¹⁰

v₂ = 4.7 × ×10⁵ m.s^-1

Semiconductors And Electronics Some Special Semiconductor Diodes

Zener Diode

When an ordinary semiconductor diode is reverse-biased, a very small saturated reverse current flows across the junction due to the flow of a few thermally-generated minority carriers (electrons in p -region and holes in n -region). This current is not at all dependent on the applied reverse bias voltage. But, if this reverse bias voltage exceeds a definite value, the reverse current increases abruptly. This situation is known as the breakdown of the semiconductor diode.

As n result, power consumed by the diode U. The rate of production of heat increases rapidly which can damage the diode. The tolerance of some specially prepared semiconductor diodes is Increased in such a way that at reverse bias, even due to the flow of high reverse current, the diode is not damaged. This type of diode has important use for practical purposes and is generally known as Zener diode

Explanation of Zener effect:

1. If the reverse bias voltage across a p -n junction diode is very high, the minority charge carriers are accelerated.

• Due to their high speed, they knock out more electrons from the covalent bonds. Such collisions produce electron-hole pairs.
• Newly generated carriers, in turn, may gain sufficient energy to disrupt more covalent bonds and produce more electron-hole pairs.
• This phenomenon is cumulative and soon an avalanche of charge carriers is produced causing a flow of large currents.
• Breakdown occuring in this manner is called avalanche breakdown and the diode is called avalanche diode.

2. If both regions of a semiconductor diode i.e., a p-n junction diode are heavily doped, the thickness of the depletion layer decreases to a large extent.

• Then a very small reverse bias is applied, and a very strong electric field is created between the two ends of the depletion layer.
• This electric field breaks up the covalent bonds of the semiconductor. crystal directly and a huge number of charge carriers are set free within the crystal.
• Tints, due to a comparatively small reverse bias, diode-breakdown occurs i.e., at a constant breakdown voltage of small magnitude, the diode reaches a state when a large reverse current flows.
• Thus breakdown that occurs in this manner is called Zener breakdown and the diode is called Zener diode
• In the case of any semiconductor diode of this type, the avalanche effect and Zener effect occur simultaneously at reverse bias.
• Generally, for a near 6 V breakdown voltage, the avalanche effect and Zener effect become equivalent to each other, and concerning temperature no special change of breakdown voltage takes place.
• So, diodes having breakdown voltage around 6 V, are very suitable to use at different temperatures.

Avalanche effect or Zener effect whichever may be die principal effect, these types of diodes are simply called Zener diodes in practical cases.

Characteristic curve:

The ampere-volt (I- V) characteristic curve of a forward-biased Zener diode is similar to that of an ordinary semiconductor diode. But, when the reverse bias voltage reaches a particular value Vz, the reverse current suddenly increases to a large value. This part of the characteristic curve is represented by AB, almost a verticle line. In an ideal Zener diode, the increase of voltage with the increase of current is zero. In practical cases, this increase is within 1 % to 5%.

Semiconductor devices class 12 notes Rating of a Zener diode:

In every Zener diode, a reference voltage and a reference power are mentioned. This voltage rating Vz indicates the reverse-bias voltage, at which the reverse current increases abruptly, but no change of the terminal voltage of the Zener diode takes place. The meaning of power rating or watt rating Pz is that, due to the increase of the current drought the diode, if the value of power consumed exceeds the j value of Pz, the diode will.be damaged

So, the maximum safe reverse current through the diode

I_max = \(\frac{P_Z}{V_Z}\) the point indicates the value.

For example, in the case of the rating 4.7 -1W of a Zener diode,

I_max = \(\frac{1 \mathrm{~W}}{4.7 \mathrm{~V}}\) = 0.21 A

= 210 mA

In the circuit for a Zener diode, a regulative resistance is so selected that the value of the reverse diode current never exceeds I_max

Circuit symbol: The circuit symbol of a Zener diode is shown below

Zener diode as a voltage regulator:

A Zener diode is used to obtain constant voltage across a load resistance connected to a fluctuating DC voltage source. The Zener diode and load resistance are to be connected in parallel.

Zener diode Working principle:

A fluctuating DC voltage source (unregulated voltage source) is connected to a Zener diode through a resistance Rs in series such that the Zener diode is reverse biased.

If the input voltage increases current through R_s and Zener diode also increases. Due to this, the voltage drop across Rs increases, but the voltage drop across the Zener diode remains constant as it operates in the breakdown region. The breakdown voltage of the Zener diode does not change by

Changing current through it. On the other hand, if the Input Voltage Is decreased, the current through R_s and Zener also decreases, but the voltage drop across Zener remains the same.

As soon as the reverse bias voltage of a Zener diode reaches V_Z despite increasing the current through it indefinitely, the terminal voltage of the diode remains constant at V_Z. Hence, the terminal voltage of a load resistance R_L connected parallel to the Zener diode also remains at V_Z, despite any change of current through it.

Conversely, it may be said that to maintain a constant potential difference across a load resistance, a Zener diode of equal voltage rating is to be connected in reverse bias parallel to the load resistance. This is called voltage regulation across a load resistance

The voltage across a Zener diode thus serves as a reference so the diode is referred to as a reference diode

Selection Of R_S:

If (V R_Z-P_Z) is the rating of the Zener diode, then the maximum safe current through it is I_Zmax  = P_Z/ V_Z. The resistance R_S is so chosen in the circuit that it restricts the Zener current below /max even for the maximum value of the unsteady input voltage.

Load regulation:

It is the capability to maintain a constant voltage (or current) level on the output channel of a power supply despite changes in the load resistance. More simply, load regulation is a measure of the ability of an output channel to remain constant for given changes in the load. In the circuit, a millimeter is connected to measure current (IL) flowing through load resistance R_L, and a voltmeter to measure the potential difference across R_L.

The changed circuit Keeping the supply voltage V_i constant, Rj is changed step by step and in ea-and V_L are recorded. Now a graph of V_L -I_L is drawn. Part AB indicates the regulated voltage. If the Zener diode behaves ideally, the line AB would become horizontal. Point B exists a bit lower. From the portion BC, it is understood that, if the magnitude of I_L is very high, i.e., the Zener current I_Z is very low, VL becomes uncontrollable. As the current I_L is maximum at the point B of the regulated zone

So, the ratio \(\frac{V_L}{I_L}\) at B indicates the minimum value of the load resistance R_L Despite fluctuation of load resistance above that minimum value through a long-range, the potential difference across the load resistance V_L remains almost constant.

If voltage at the point A is V_NL (NL means no load or zero current) and voltage at the point B is V_L,

Then percentage regulation \(\frac{V_{N L}-V_L}{V_{N L}} \times 100 \%\)

In an ideal Zener diode, this percentage regulation is zero and in actual practice, this value lies within 1% to 5%.

Light Emitting Diode or LED

If a specially made semiconductor diode or p-n junction forward bias emits light spontaneously, it is known as light-emitting, diode, or LED.

Silicon; (Si) or Germanium (Ge) diode is unsuitable as LED. For LED, semiconductor crystals of Gallium arsenide (GaAs), Gallium Phosphide (GaP), Silicon Carbide (SiC) etc. are used. The color of the light emitted from LED depends on the band gap of the semiconductor crystal and the strength of doping.

LED Working principle:

When a p-n junction diode is forward-biased, both the electrons and the holes move towards the f junction. As they cross the junction, recombination of a few electrons and holes takes place and energy is released at the junction in the form of light. Photons are emitted from the p-n junction. The color of the emitted light depends on the energy of the photons.

From the principle of conservation of momen¬ tum, it is found that a photon can be emitted only when an elec¬ tron and a hole combine with equal and opposite momentum. This condition is fulfilled in some crystals like GaP or SiC but not in Ge or Si. In the latter, the released energy of the electron-hole pair is converted to heat energy which only makes the crystal heated. For this, GaP or SiC-like crystals rather than Ge or Si are generally used to construct LED.

LED Circuit Symbol:

With the symbol bf ordinary semi ductor diode two arrows directed outwards1′ are drawn. This indicates the circuit symbol of the LED

LED Characteristic curve:

The volt-ampere characteristic curve of LED is identical to that of an ordinary semiconductor diode. But when it is forward biased, due to emission of light, a few electron-hole pairs are current (/in mA) destroyed. So, the magnitude of the current is less than that of an -ordinary diode.

But, by the low current, the action of LED is not hampered because forward bias never exceeds 2.5V or 3V and the maximum value of .current in forward bias does not exceed 50mA. If the magnitude of the forward current is increased slowly from 10mA to 50mA, the intensity of light emitted from LED continuously increases

LED Uses:

The power consumed by an LED is very small. In a well-planned circuit, these are not easily damaged and can be used uniformly for a long time. Moreover, LEDs are cheap. Hence, LEDs are extensively used in electrical and electronic circuits at present. It is extensively used for fast on-off switching. Besides these, LEDs are used in various electronic circuits, like torchlights, low-power household electric lamps, calculator digital watches, etc. These diodes are also used in signal lamps

WBCHSE physics semiconductor electronics Photodiode

A photodiode is a special type of reverse-biased semiconductor diode. If the light is made to fall on its p-n junction, the reverse saturated r current increases almost linearly | with the intensity of the incident I light. The circuit diagram of a photodiode.

The reverse bias of the junction diode, naturally a small reverse current flows in the circuit. This is called dark current Now light is made to fall on p -n junction through a lens. New electron-hole pairs are created in exchange for the energy: of the incident photons and hence the reverse current increases. It is found that, the

The magnitude of the reverse current is proportional to the Intensity of the incident light. But. if the energy of the photon of the incident light is not sufficient to create additional electron-hole pairs, the photodiode will not function

The volt-ampere characteristics of a photodiode Are Shown In

Photodiodes are used for the identification of sound from sound or sound-track of cinema, determination of the Intensity of light, light-operated switches, electronic counters, CD players, smoke detectors, etc

Photodiode Circuit symbol:

With the symbol of an ordinary semicon¬ductor diode, two arrows directed inward are drawn. This indicates the circuit symbol of the photodiode.

WBCHSE physics semiconductor electronics 

Semiconductors And Electronics Solar Cell

A special and very important practical application of photodiode is solar cells. In a photodiode, incident solar energy Is so converted into electrical energy that, it behaves as a battery. In the daytime In the presence of sunlight, this solar battery is used as a charger. Afterward, this storage battery is used to operate various electrical appliances. Solar cells are used in artificial satellites or space vehicles to operate various electrical instruments kept Inside the satellite or space vehicles. Also, solar cells are used in calculators

Earth’s surface gets an average of 1000 W of solar power per square meter on a sunny day. Only 10% of the incident photons can produce an electron-hole pair and make a photodiode active. So, approximately 100 W of solar power per square meter is available for transformation to electrical energy. This available energy is too small compared to approximate generally the available solar energy is focused on a small area with the help of a concave mirror. But due to this process, the temperature of the photodiode increases so much that even the much

Effective silicon crystal loses its efficiency. So in this case, the use of semiconductor crystals like Gallium Arsenide (GaAs) is suitable. reverse bias voltage more

The characteristic curve of the photodiode lying in the 4th quadrant is very relevant to, the action of a solar cell. In this case, a potential difference is positive i.e., p-n

Junction is in forward bias. But current is negative i.e. reverse current flows through the junction. It is to be noted that, current flows through the pn junction from the negative end to the positive end and this flow Is Identical to the flow of current through a battery, So the photodiode l.e„ the solar diode behaves like a cell or battery. The forward bias voltage of a cell does not exceed

1 V and the magnitude of reverse current Is very small, hence to increase output power, the internal resistance of a cell Is made very small. The output voltage is increased by connecting a large number of cells connected in series and the output current Is also increased by a large number of such series combinations in parallel. The details of technology regarding the construction of a solar cell are beyond our present discussion.

WBCHSE physics semiconductor electronics 

Semiconductors And Electronics Junction Transistor

In 1947, John Bardeen, William Shockley, and Walter Brattain Invented the transistor Out of different forms of translators, the most widely used form Is the bipolar Junction transistor (BJT), It Is a semiconductor device In which the current How between the two end terminals (called the collector and the emitter), Is controlled by an amount of current following through an Intermediate third terminal (called the base).

Transistors are used In almost all modern technologies. Thus, the Importance of semiconductors Is Immense In the modern age, hence, the modern age Is also known as ‘the silicon age Like a diode, a transistor Is also made from a crystal. It Is very small In size and Is kept sealed Inside a metallic or plastic covering In such a way that It cannot come in contact with air or moisture

Transistors are of two types:

1. p-n-p transistor and
2. n-p-n transistor

Structure of a p-n-p transistor:

1. A thin n-type layer Is introduced by doping between two p-type regions at the two ends of a semiconducting crystal. The n-type layer at the middle is very small in thickness in comparison with the two p-type regions at the two ends, This layer at the middle is known as the base (B) of the transistor

2. The two p-type regions at the two ends of the transistor are called emitter (E) and collector (C). Although these two regions are identical, the rates of doping them are different. The emitter region is heavily doped compared to the collector region. Hence, in a circuit, if the connection of the emitter and collector is interchanged, the working of a transistor gets disturbed.

3. The rate of doping of the base of a transistor is much less than that of its emitter and collector. CID The majority of charge carriers of this kind of transistor are holes. Usually, holes are emitted from the emitter and after crossing the thin layer of the base, they are collected by the collector. The thin layer of the base controls this flow of holes.

Structure of an n-p-n transistor:

The structure of an n-p-n transistor Is almost identical to that of a p-n-p transistor, as discussed above. For the n-p-n transistor, the failure of doping of the different parts of the die semiconducting crystal Is Just the opposite of the p-n-p transistor. In this case,

• The thin base layer at the middle is of p-type
• The emitter and collector regions at the two ends are of n-type
• For this kind of doping, the. majority charge carriers electrons.

The speed of electrons Is more than that of holes as charge carriers. Mdnce In high-frequency circuits and computer circuits, n-p-n transistors are used. Actually, during the transmission of signals through these circuits, the greater the speed of the effective charge carriers, the greater will be the rate of work done.

p-n-p transistor and n-p-n transistor Circuit symbol:

The circuit symbols of p-n-p and n-p-n transistors are. The arrow sign indicates the? direction of conventional current flow between the emitter and the base. So, electrons flow in the direction opposite to the arrow sign

Transistor In an open circuit:

Let us assume that a transistor Is a combination of two diodes. So, diffusion of electrons and holes takes place through the junctions just like that of a p-n junction diode, As a result, each p-n junction becomes reverse-biased without the presence of any external source depletion region

Just as In a p-n junction diode, depletion regions are formed around the Junctions in a p-n-p transistor

Common-Emitter or CE Configuration of a Transistor

Three, kinds of circuit carts be constructed using transistors:

1. Common-base (CB)
2. Common-emitter (CE) and
3. Common-collector, (CC)

Among these, a common emitter or CE circuit is widely used as an amplifier circuit

The flow of charge carriers In a CE-cIrcuit:

We take a n-p-n transistor and consider the flow of conduction electrons

It is to be noted that, by convention, the direction of currents is opposite to that of the moving electrons.

Real-Life Scenarios Involving Semiconductor Devices

A CE circuit is shown using an n-p-n transistor. In this case:

1. The circuit connecting the base and the emitter (left side circuit in the figure) is used as the input circuit and
2. The circuit connecting the collector and the emitter (right side circuit in the figure) is used as the output circuit. Hence, in both circuits, the emitter is common. In an alternating current (AC) circuit this emitter is grounded. So it is also called a grounded emitter circuit

Biasing of a CE circuit:

1. Keeping the emitter grounded, the base is kept at forward bias in the input circuit, i.e., the p-type base of the n-p-n transistor is connected with the positive pole of the source battery V_BB
2. Keeping the emitter grounded, the collector is kept at reverse bias in the output circuit, i.e., the n-type collector of the n-p-n transistor is connected with the positive pole of the I source battery V_CC

Current in a CE circuit: 

In a tile input circuit, the emitter is at the negative potential concerning the base. Hence, a large number of majority carriers, i.e., electrons are emitted from the tire emitter which is then attracted by the positive base. Since the base layer is very thin, most of these moving electrons enter the collector after crossing the thin base layer. Then they are attracted by the positive potential of the collector.

The small number of electrons that fail to cross the base are attracted by the positive potential of the base. In this way,

The flow of electrons from the emitter produces two currents:

1. The base current of the input circuit and
2. The collector current of the output circuit.

The conventional direction of electric current is opposite to the direction of electron flow. According to that, the emitter current I_E base current I_C and collector current 

Clearly, I_E = I_B+ I_C

The value of I_B is much less than I_E or I_C

For example I_B = 10μA , I_C = 2mA = 2000μA

Then, I_E = 2000 + 10 = 2010 μA

Discussions:

• Keeping I_E at a fixed value, if I_B is increased, then from the relation I_C = I_E– I_B, we see that the value of I_C decreases. Hence in the CE circuit, the phase difference between the output signal and the input signal is 180°.
• Usually, the power expended in the output circuit of a transistor is much greater than that of the input circuit. Hence for commercial purposes the area of the base-collector junc¬ tion of a transistor is made much greater than the emitter-base junction
• As the emitter-base is forward-biased, the input resistance, i.e., the resistance of the emitter-base junction becomes very small. Again, as the collector-base junction is reverse biased, the output resistance, i.e., the resistance of the emitter-collector junction becomes very high. The circuit with IowTinput resistance and high output resistance acts as the best current amplifier

CE characteristics:

• Source voltage of the input circuit= V_BB
• Source voltage of the output circuit = V_CC
• Base current, I_B = input current
• Base-emitter voltage, V_BE = input voltage
• Collector current, I_C = output current
• Collector-emitter voltage, V_CE = output voltage

Among the input and output currents and voltages, only the input current I_B and output voltage V_CE can be changed easily according to need, i.e., in a CE circuit IB and V_CE should be taken as independent variables, and V_BE as well as I_C as the two dependent functions of them. Out of these, V_BE has less importance in the analysis of the circuit

So, I_C = f(I_B,V_CE) ………………………………………. (1)

Using the mathematical relation (1), two characteristic curves of the CE circuit can be drawn:

WBCHSE physics semiconductor electronics 

Transfer characteristics:

Keeping V_CE the graph of Ic drawn concerning I_C is known as transfer characteristics. In this case, I_B and I_C are the input and output quantities respectively.

Generally, I_C changes linearly with I_B

The ratio Δ I_C /Δ I_B is called current transfer ratio current amplification factor or current gain it is expressed by the symbol β Usually, the range of β is 100 to 500, approximately

Output characteristics:

Keeping I_B at different fixed values, the graphs drawn concerning V_CE are called output characteristics. In this case, both V_CE and Ic are output quantities. For different values of I_B, a series of different output characteristics is obtained.

This series is divided into three clear regions:

1. Active region: In this region, the base-emitter junction is forward-biased and the collector-emitter junction is reverse-biased. As a result, I_B> 0 and V_CE> 0; but in actual practice, the value of V_CE should be more than 0.2V (approximately) to keep the collector junction in the actual reverse bias. If a transistor is to be used as a good amplifier without much distortion, it has to be operated in the active region.
2. Cut-off region: In this region, both the base-emitter and the collector-emitter junctions are reverse-biased.
3. Saturation region: In this region, both the base-emitter and the base-collector junctions are forward-biased. Remember that, if the value of V_CE is less than 0.2 V (approximately), the collector is forward-based effectively.

Use of transistor as a switch:

An Ideal switch, when It is made ‘on: makes a circuit closed, On the other hand, when It is made ‘off,’ the circuit becomes an open one. Moreover, all these are done by an Ideal switch momentarily, No transistor can satisfy these conditions of an Ideal switch properly. Despite that, on the whole, the use of a transistor as a switch in different electronic circuits Is very wide.

If a transistor is employed in common-emitter mode, In the cutoff region the base-emitter junction Is reverse-biased. Under this condition, base current lB Is negative and the magnitude of the collector current is very small. This Is called the ‘off’ condition of the transistor.

On the other hand, if the base current attains a high positive value, the collector-emitter junction is forward-biased and the transistor is placed In the saturation region. In this case, the collector-emitter voltage Is nearly zero. So, almost the whole external bias Vcc acts as the terminal potential difference of the load resistance RL.

So, the collector current Ic reaches a sufficiently high value. This condition is treated as the ‘on’ condition of the transistor. In a switch system, the arrangement is to be made to turn the base current of the transistor from a positive to a negative value or from a negative to a positive value very rapidly. As a result, the transistor can turn from ‘on’ to ‘off’ or from ‘off’ to ‘on’ respectively.

But when it is ‘on,’ the collector current Ic takes some time to reach a high value and when it is ‘off; the charge collected at the base takes some time to decay. Hence, a transistor as a switch can never act with the rapidity of an ideal switch. Only its efficiency can be increased by using some specially designed transistors.

Accordingly, since a transistor can be in either ‘on’ mode or ‘off’ mode, it has considerable use in digital circuits. The application of transistors to make NOT logic gates has been discussed in the chapter ‘Digital Circuits!

Use of transistor as a current amplifier:

In the circuit, DC biases, VBB, and Vcc have been applied

At the base-emitter junction and collector-emitter junction of an n- p-n transistor respectively, R₁ is the load resistance, In the collector-emitter circuit

V_CC = V_CR + I_CR_L

[V_CR is de voltage and I_C is DC]

In the cut-off region, I≈0, So, V_CR ≈ V_CC The point A indicates this condition.

Again In the saturation region, V_CE ≈  0, So, V_CC≈ I_CR_L Or, \(\frac{v_{C C}}{n_L}\). Point B In, indicates this condition.

The line AB is called the load line of the referred circuit. If a transistor Is used as a current amplifier in common-emitter mode, DC bias voltage V_BB and V_CC and load resistance R_L are so selected that, the action of the transistor is confined in the active region.

Under this condition, if the output characteristics for the constant base current I_B intersect the load line at point Q.

• This point is called the DC operating point or, Q -point of the circuit. In the case of an amplifier circuit, generally, a weak AC signal is supplied across the base-emitter circuit as input.
• For example: If a sound is made in front of a microphone, a weak AC signal is obtained. To apply this AC signal to a DC circuit, a condenser Cx is used.
• Direct current (dc) cannot pass through the condenser C, the input signal is free from the influence of the battery V_BB. The input AC signal is added to the constant dc base-cur¬ rent IB. So, the base current oscillates between I_B1 and I_B2
• The point Q oscillates between P and R. It is understood easily from the output characteristics that, the collector current Ic oscillates between I_C1 and I_C2.
• So, the AC signal obtained is the output signal. the values of I_B are generally expressed in the microampere {μA) scale and the values of I_C are expressed in the milliampere (mA) scale.
• So, the amplitude of the output signal is greater than that of the input signal by 100 to 500 times. This is the current amplification by the transistor. Only the ac part of the amplified output signal is taken out from the two ends of the load resistance R_L with the help of the Condensor C₂ .’
• The two parts of the circuit containing C₁ and C₂ are called filter circuits. From the mixture of AC and DC, the condensers filter outin’ AC stopping the DC. The output AC signal can be applied again as an input signal in the amplifier circuit of another transistor. Hence, the signal is again amplified.
• Thus, using successive amplifier circuits, input AC signals of small amplitude can be amplified many times.

But this type of magnification has a limit. If the magnification is very large, the waveform of the output ac is distorted. The output waveform does not resemble the input waveform. In that case, the output signal becomes useless. For example, if a man speaks out in a low voice in front of a microphone, the outcoming speech from the loudspeaker becomes distorted and hard to understand

Current amplification factor or current gain:

In the CE circuit, the current amplification factor of current gain is defined as the ratio of a component of the output collector current to a component of the input base current. It is denoted by the symbol 0. In different types of. transistors, the value of 0 is in the range 20 to 200.

If i_b = input ac base current and is = output ac collector current, then

β = \(\frac{i_c}{i_b}\) ……………………(1)

Initially, a stable DC biasing is applied in each transistor circuit. Now an input AC signal is applied at the base of the transistor. If Jg and IQ are dc base current and dc collector current respectively, then at any instant, total base current =I_B + i_b and total collector current = I_C+i_c

Both AC currents i_b and i_c are considered instantaneous changes in constant DC currents IB and Ic respectively. Hence we can write, i_b = ΔI_B and ic = ΔI_C. Therefore current amplification can also be written as

β = \(\left(\frac{\Delta I_C}{\Delta I_B}\right)_{V_{C E}}\) ……………..(2)

On the other hand, the ratio of change in collector current to the change in emitter current IE is known as the current transfer ratio of the transistor and it is denoted by the symbol a.

α – \(\left(\frac{\Delta I_C}{\Delta I_E}\right)_{Y_{O B}}\) constant) ……………..(3)

Here, I_C< I_E Since in general, the value of ΔI_C< ΔI_E   Since in general, the value of I_EB is very small, so the value .of α is less than unity but still very nearly equal to 1, α≈ 1 (In most of the transistors, a is in the range 0.95 to 0.995 approximately). In a CB circuit, this parameter is of great importance, but almost irrelevant to a CE circuit.

Relation between α and β: Now from equation (2) we have,

β =  \(\frac{\Delta L_C}{\Delta I_B}=\frac{\Delta I_C}{\Delta I_E-\Delta I_C}\)

β =  \(\frac{\Delta I_C / \Delta I_E}{1-\Delta I_C^* \cdot \Delta I_E}\)

Or,  β =  \(\frac{\alpha}{1-\alpha}\)

∴ α = \(\frac{\Delta I_C}{\Delta I_E}\)

For example, if = 0.995

β = \(\frac{0.995}{1-0.995}=\frac{0.995}{0.005}\) ≈200

And if. α  = 0.995

β  = \(\frac{0.95}{\mathrm{I}-0.95}=\frac{0.95}{0.05}\)≈ 20

Voltage gain and Power gain:

For the CE circuit, if ΔV_i is the change in input voltage” and  ΔV₀ is the corresponding  output voltage, then

Voltage again   = \(\frac{\Delta V_o}{\Delta V_i}=\frac{\Delta V_{C E}}{\Delta V_{B E}^*}\)

= \(\frac{\Delta I_C R_L}{\Delta I_B R_B}=\beta \frac{R_L}{R_B}\)

Here, R_L = Load resistance

And R_B = Base resistance or input resistance

Power again = \(\frac{\Delta P_o}{\Delta P_i}=\frac{\Delta V_{C E} \cdot \Delta I_C}{\Delta V_{B E} \cdot \Delta I_B}\)

= \(\beta \frac{R_L}{R_B} \cdot \beta=\beta^2 \frac{R_L}{R_B}\)

Power gain =’ current gain ×  voltage gain

Another parameter, called transfer conductance or transconductance g_m is defined as g_m  = \(\frac{\Delta I_C}{\Delta V_{B E}}\)

Semiconductors And Electronics Junction Transistor Numerical Examples

Example 1. In a common-emitter circuit, the collector-emitter voltage is fixed at 5V. For base currents 30 μA and 40 μA, the collector currents are 8.2 mA and 9.4 mA respectively. Calculate the current gain of the circuit
Solution:

The change in base current

ΔI_B  =(40- 30)μA = 10 μA

The change in the collector’s current

ΔI_C = (9.4-8.2) = 1.2 mA = 1.2 × 10³ μA = 1200 μA

∴ β = \(\frac{\Delta I_C}{\Delta I_B}\)

= \(\frac{1200 \mu \mathrm{A}}{10 \mu \mathrm{A}}\)

= 120

Examples of Applications of Semiconductors in Technology

Example 2. The collector current of an n-p-n transistor is 10 mA. If 99.5% of the emitted electrons reach the collector, determine the emitter current, base current, and amplification factor of the transistor.
Answer:

In an n-p-n transistor,  we know collector current =I_C= 10 mA, emitter current = I_E, base current = I_B  and

Amplification factor, β =  \(\frac{I_C}{I_B}\)

According to the problem, I_C = 99.5% of I_E

∴ I_C  = \(\frac{995}{1000} I_E\)

Or,  I_E = \(\frac{1000}{995} \times 10\)

= 10.05 mA

I_B = I_E – I_C= (10.05 -10) mA

= 0.05 mA

β = \(\frac{I_C}{I_B}=\frac{10}{0.05}\)

= \(\frac{10}{0.05}\)

= 200

Example 3.  An n-p-n transistor is kept in a common-emitter configuration. The amplification factor of the transistor is 100. If the collector current is changed by 1 mA, what will be the corresponding change in the emitter’s current?
Solution:

Amplification factor, β = \(\frac{\Delta I_C}{\Delta I_B}\)

ΔI_B = \(\frac{\Delta I_C}{\beta}\)

= \(\frac{1}{100}\)

= 0.01 mA

So, change in emitter current,

ΔI_E= ΔI_C+ΔI_B = 1+0.01

= 1.01 mA

Conceptual Questions on Intrinsic and Extrinsic Semiconductors

Example 4. The input resistance of a silicon transistor is 100 Ω. Base current is changed by 40 μA which results in a change in collector current by 2mA. This transistor is used as a common emitter amplifier with a load resistance of 4kΩ. What is the voltage gain of the amplifier?
Solution:

Voltage gain \(=\frac{\Delta V_o}{\Delta V_i}=\frac{\Delta I_C R_L}{\Delta I_B R_B}=\beta \frac{R_i}{R_1}\)

Load resistance R_L= 4 kΩ = 4000Ω

Input resistance R_B = 100Ω

∴ Current gain = \(\frac{\Delta I_C}{\Delta I_R}=\frac{2 \mathrm{~mA}}{40 \mu \mathrm{A}}\)

= \(\frac{2 \times 10^3}{40}\)

= 5

∴ Voltage gain = \(50 \times \frac{4000}{100}\)

= 2000

Semiconductors And Electronics Oscillators

Oscillator Definition:

The system that can convert a DC or unregulated AC signal to an AC signal of a certain frequency is called an oscillator

Feedback: Let A be the amplification of a voltage amplifier. If the input and output voltages are V and VQ respectively, then

A = \(\frac{V_o}{V_s}\)

i.e  V₀ = AV_s …………………………………………….(1)

Now a feedback circuit is connected between the points P and Q in this amplifier circuit. The voltage between P and Q is so controlled that a part of the output voltage V₀ (say, βV₀) is again fed back to the input through the feedback circuit.

This phenomenon is known as feedback. β is known as the feedback ratio, where 0 < 1.

In this case, the effective input voltage of the amplifier circuit

V_i= V_s= βV₀

So, the output voltage,

V₀= AV_i = A(V_s+βV₀) = AV_s+βV₀

Or, V₀-AβV₀ = AV_s

Or, V₀{1-Aβ) = AV_s

So, the effective amplification of the amplifier circuit

A_s = \(\frac{V_o}{V_s}=\frac{A}{1-A \beta}\) ……………..(2)

In general, the self-amplification A of the amplifier is called open loop gain and the effective amplification A_f due to feedback is called closed loop gain. Gain A_f is known as loop gain.

Negative feedback:

If loop gain Aβ is real and negative, then according to equation (2) (1-Aβ) > 1 and A_f< A.

Due to such feedback, the effective amplification Aj becomes less compared to self-amplification A. This is called negative feedback.

Despite the lowering of amplification, negative feedback has great utility due to some special advantages:

• The amplification can be kept at a stable value.
• The distortion in the output signal concerning the & put signal can be removed.
• The internal noises of the. the amplifier can be minimized.
• The effective bandwidth [see the chapter ‘Communication System’] increases and so on.

Positive feedback:

Barkhausen criterion: if the loop gain A₀ is real, positive, and less than 1, then (1-Aβ) <1 and A_f>A. Consequently, the effective amplification A_f becomes greater concerning self-amplification A of the amplifier. It is called positive feedback.

Generally, the reactive components, like inductors or capacitors are used in feedback and amplifier circuits. As a result, A and 0 both become complex, instead of being real, which means that | an addition to the numerical values, ‘these quantities include a phase factor

Let us assume that the components of an amplifier and positive feedback circuit are so chosen that the following condition Is satisfied

Then from equation (2), A_f = ∞, i.e., the effective amplification of the amplifier becomes infinity.

• Hence, the amplifier produces an output signal without any externally applied Input signal. Thus, the amplifier becomes an oscillator
• This condition is called the Barkhausen criterion of oscillation. This condition means that |Aβ| = 1 and the phase difference for a complete feedback cycle is zero or an integral multiple of 2n.
• If the components used in the feedback circuit remain unchanged, then it is observed that for a certain frequency (say f₀ ), the condition (3) is satisfied. Only for this specific frequency (say f₀), the magnitude and phase of the feedback voltage become equal to those of the input voltage.
• So, the feedback voltage itself itself effectively an input signal. Hence, no external input signal is required to obtain an output signal.
• Thus, an oscillator can generate an output signal of a particular frequency without any externally applied input signal.
• Due to this, an oscillator may also be called a self-sustaining device. Output can be generated without input this is true for the signal only.
• Given the law of conservation of energy, to get a stable alternating voltage or alternating current of a specific frequency as an output, we should connect an energy source to the input.
• Generally, any DC source or AC source of unregulated frequency is used for this purpose

Oscillators Classification 

Depending on the active arrangements of components to generate oscillation,

Oscillators can be classified as:

1. Feedback oscillators and
2. Negative resistance oscillators.

On the other hand, according to the range of frequencies generated by an oscillator,

It can also be classified as an:

1. Audio frequency oscillator or AF oscillator,
2. Radiofrequency oscillator or RF oscillator etc.

In the case of sinusoidal oscillators, depending on the particular circuit used as the frequency-determining circuit, oscillators are named as LC oscillators, RC oscillators, crystal oscillators, etc.

LC Feedback oscillator:

In the feedback amplifier, an LC circuit has been used as a feedback circuit.

From AC analysis, it will be observed that such a type of circuit will generate alternating voltage or alternating current of a constant frequency. The frequency is given by

f₀ = \(\frac{1}{2 \pi \sqrt{L C}}\)  ……………………………………. (4)

Now, if a DC source is applied at the input, it can be observed that such type of DC source, whatever may be its stability in magnitude, always contains some amount of distortions or ripples mixed with it. These are called noise. Ripples of each noise can also be analyzed as a combination of many sinusoidal waves. Each of such sinusoidal waves reaches the output point Q after getting amplified by the amplifier A.

Then the LC feedback circuit brings back the wave of frequency fQ [as shown in equation (4) ] to the point P. For all other frequencies except fQ, LC circuits act as rejector circuits. Hence, no feedback of the frequencies other than f₀ takes place to the input.

• Thus the wave of frequency f₀ undergoes repeated feedback and amplification and ultimately attains stability at the output. Outputs for all other frequencies become negligible.
• So, it can be said that the LC feedback oscillator generates an alternating wave of a constant frequency f₀. This frequency-determining LC circuit is called a tank circuit.
• It may be noted that the noise at the input is the source of the output wave of a constant frequency.
• If the magnitude of the components of the LC circuit is changed, then according to equation (4), the magnitude of fQ will also be changed. Thus by changing the magnitudes of the components of the LC circuit, alternating waves of other frequencies can also be generated.
• Particularly, if it is so arranged that the magnitude of capacitor C can be changed continuously, then the LC feedback oscillator is converted to a variable frequency oscillator.

Besides generating sinusoidal waves, if an oscillator is used to generate square waves, triangular waves, and other types of complex waves, then this oscillator is termed a multivibrator

Designing of an oscillator using a transistor amplifier:

How an n-p-n transistor can be used as an oscillator (Resistors which are used for biasing of the transistor have not been shown here)

In this oscillator, a frequency-determining tank circuit has been used. This tank circuit is a combination of a capacitor C and a mutual inductor M (L and L’ are constituent selfinductors of this mutual inductor). This combination of C and M acts as the feedback circuit across collector output and base input. We know, that in the case of the common-emitter (CE) configuration of the transistor, there is a phase difference of 180° between input and output.

The components of the tank circuit are so selected that due to feedback, it again generates a phase shift of 180°, which means the feedback voltage is in the same phase as the input volt¬ age. Such an oscillator is called a tuned collector oscillator. Generally, this type of oscillator is used for generating alternating output of high frequency of the order of 1 MHz. There are some other varieties of oscillators made of transistors which find different applications.

In the case of reverse biasing, the net flow of holes is from n -region to p -region. Because, in this case, the majority carrier holes in the p -p-region cannot enter the n -n-region, but the minority carrier holes can move easily from the n -n-region to the p -p-region

 

Semiconductors And Electronics Synopsis

1. The substances having electrical conductivity intermediate between conductors and insulators are called semiconduc¬ tors. Examples: silicon, germanium, etc.

2. If any electron is released from the bond of an atom, the deficit of electrons appears at that position is known as a hole. Its effective charge is +e, although it is not a real particle.

3. Innumerable energy levels which remain very close to each other, form an energy band.

4. The separation between two consecutive energy bands in a solid is called the forbidden band or forbidden zone. No electrons can stay in the forbidden zone.

5. Electrons residing at the highest energy band in an atom are called valence electrons. The energy band that is formed by the energy levels in which the valence electrons of a substance can reside, is called the valence band. The energy levels possessed by the free electrons or conduction electrons of a substance constitute the band known as the conduction band.

6. The energy difference between the conduction band and the valence band is called the energy gap or band gap. If the energy of the conduction band is Ec, the energy of the valence band is Ev and the band gap is Eg, then, Eg = Ec-Ev.

7. In the case of insulators, the energy gap between the valence band and conduction band is very large

8. In the case of conductors, the upper portion of the valence band overlaps with the lower portion of the conduction band.

9. In the case of semiconductors, the energy gap between the valence band and the conduction band is small.

10. In the case of an intrinsic semiconductor, the number of electrons in the conduction band and the number of holes in the valence band are equal.

11. If some special type of impurities are mixed with the intrinsic semiconductor in a controlled manner, the conduction of the semiconductor increases manyfold.

This type of semi¬ conductor is known as an extrinsic semiconductor. The method of mixing impurities is called doping. The impurities thus mixed are called dopants.

12. If pentavalent (group V) elements (like arsenic or phosphorus) are doped as impurities in the crystal of an intrinsic semiconductor (like Si or Ge) in a controlled manner, the crystal thus formed is called an n-type semiconductor. Its majority carriers are electrons.

13. Phosphorus or arsenic supplies free electrons to the intrinsic semiconductor crystal and hence they are called donors.

14. If trivalent (group III) elements (like boron or aluminum) are doped as impurities in the crystal of an intrinsic semi¬ conductor in a proper well-controlled manner, the crystal thus formed is called a p-type semiconductor. Its majority of carriers are holes

15. Boron or aluminium when mixed the pure crystal, produces holes in their bonding and can accept electrons. Hence, they are called acceptors.

16. By the opposite kind of doping, if one part of a semiconductor crystal is made of p-type and the other part of n-type, then that crystal is called a p-n junction or semiconduc¬ tor diode.

17. The connection of the electrical components like a diode, transistor, etc., with an external source of electricity

For example:  A battery), is called biasing.

18. To apply forward bias to a p-n junction, its p-end and n-end are connected with the positive and negative terminal of the external electric source respectively.

19. Reverse bias is applied to a p-n junction by connecting the n-end of the junction with the positive terminal of the external source and the end with the negative terminal.

20. The variation of current with potential difference applied to a p-n junction diode in its forward or reverse biased condition is known as I- V characteristics or simply the characteristic curve of a p-n junction.

21. The arrangement that converts an alternating waveform into a unidirectional waveform,

For example: An alternating current into a unidirectional current is called a rectifier.  For the rectification of an alternating current, p-n junc¬ tion diodes are widely used.

22. The most used form of a transistor is a bipolar junction transistor (BIT). It is a semiconductor device containing

23. Three terminals or connecting points (base, emitter, and collector)

24. In a p-n-p and an n-p-n transistor, the majority of charge carriers are holes and electrons respectively.

Three kinds of circuits can be made by using transistors:

1. Common-base (CB),
2. (U) Common-emitter (CE) and
3. Common-collector (CC)

As an amplifier circuit, CE configuration is widely used.

25. In using a transistor In the CE mode,

The circuit connecting the base and the emitter is the input circuit

The circuit connecting the collector and the emitter is the output circuit.

Emitter is common and in an AC circuit, the emitter is grounded.

26. In the CE mode, By keeping the emitter grounded, the base is forward-biased in the input circuit.

By keeping the emitter grounded, the collector is = reverse-biased in the output circuit.

27. The output characteristic curves of a transistor have three regions:

Active region,

Cut-offregionand

Saturation region.

28. The system that can convert a DC or unregulated AC signal to a signal of a certain frequency Is called an oscillator.

29. The circuit of the feedback amplifier is designed in such a way that the effective amplification of the amplifier reaches infinity.

30. In a semiconductor,

• Current density, J = e(nv_e +pv_h)
• And conductivity, cr = e(nμ_e+ pμ_e)
• where,n = number density of electrons
• p = number density of holes;
• v_e = drift velocity of electrons;
• v_e = drift velocity of holes;
• μ_e= mobility of electrons;
• μ_h = mobility of holes.

31. For n-type semiconductor, n > p, and for p-type But in all cases, np = n²_i

32.Energy of the forbidden gap, E_g = hc/ λ_max   [where λ_max = the corresponding maximum wavelength of the forbidden gap]

33. If V (volt)- P (watt) Is the rating of a Zener diode, then the maximum safe current through the Zener diode as a voltage

34. If the emitter current is lp, the base current be, and the collector current is l_C, then in the case of the V_CE mode of a transistor

I_E= I_B +I_C

I_C= f(I_B,V_CE)

Current transfer ratio, α = \(\frac{\Delta I_C}{\Delta I_E}\)

current amplification factor β = \(\frac{\Delta I_C}{\Delta I_B}=\frac{a}{1-a}\)

In A feedback oscillator, if an LC circuit is used as a back circuit, then

Output frequency of the oscillator = \(\frac{1}{2 \pi \sqrt{L C}}\)

Semiconductors And Electronics Very Short Questions And Answers

Question 1. What type of impurity is required to prepare an n-type semiconductor?
Answer: Pentavalent element

Question 2. What type of impurity is required to prepare a p-type semiconductor?
Answer: Trivalent Element

Question 3. What kind of semiconductor will be produced if it is doped with a donor element?
Answer: n – Type

Question 4. What is the effective electric charge of a hole?
Answer: +e

Question 5. The total number of negative charge carriers in an intrinsic semiconductor is n. What is the total number of positive charge carriers in this semiconductor?
Answer: n, because the total number of positive and negative charge carriers are equal]

Question 6. What change in the energy band gap of a pure semicon¬ ductor occurs due to an increase in temperature?
Answer: Remains the same

Question 7. What change in the energy band gap of a semiconductor occurs due to an increase in doping?
Answer: Decrease

Question 8. At which temperature is a semiconductor completely transformed into an insulator?
Answer: 0K

Question 9. What kind of semiconductor will be produced if a silicon crystal is doped with arsenic?
Answer: n-type

Question 10. If a frill-wave rectifier draws input from a 50 Hz main, what will be the ripple frequency of the output?
Answer: 100 Hz

Question 11. What will be the change in the thickness of the depletion region, if a p-n junction is forward-biased?
Answer: Thickness will decrease]

Question 12. In which condition, does a semiconductor diode behave like an open switch?
Answer: In reverse biasing

Question 13. What kind of biasing is required to use a Zener diode as a
Answer: Reverse biasing

Question 14. What type of biasing gives a semiconductor diode very high resistance?
Answer: Reverse biasing

Question 15. Mention the practical importance of a Zener diode in the laboratory.
Answer: As a voltage regulator

Question 16. Under what condition does a p-n junction diode work as 1 an open switch?
Answer: At reverse bias

Question 17. Write the two processes that take place in the formation of a p-n junction.
Answer: The two processes that take place In the formation of a p-n junction are diffusion and drift

Question 18. Name two important processes that occur during the formation of aap-n junction.
Answer: The two important processes that occur during the formation of a p-n junction are diffusion and drift

Question 19. What are the majority carriers in ap-type semiconductors?
Answer: Holes are the majority carriers in p-type semiconductor

Question 20. What type of semiconductor is produced If germanium crystal is doped with arsenic?
Answer: An n-type semiconductor is produced if germanium crystal Is doped with arsenic.

Question 21. Name the junction diode whose 1-V characteristics are drawn below
Answer: The junction diode is a solar cell

Semiconductors And Electronics Assertion Type

Direction: These questions have statement 1 and statement 2 Of the four choices given below, choose the one that best describes the two statements.

1. Statement 1 is true, statement 2 is true; statement 2 is a correct explanation for statement 1.
2. Statement 1 is true, and statement is true; statement 2 is not a correct explanation for statement 1.
3. Statement 1 is true, and statement 2 is false.
4. Statement 1 is false, and statement 2 is true

Question 1.

Statement 1: The depletion layer is also generated at the junction of ap-n a junction diode without any applied biasing.

Statement 2: The diffusion of thermal electrons and holes takes place from one region to another.

Answer: 2. Statement 1 is true, and the statement is true; statement 2 is not a correct explanation for statement 1.

Question 2.

Statement 1: The holes are created in the valence band only if the electrons from the valence band transit to the conduction band.

Statement 2: Due to the applied electric field, the hole in a semiconductor gains velocity which is less than that of a free electron.

Answer: 2. Statement 1 is true, and the statement is true; statement 2 is not a correct explanation for statement 1.

Question 3.

Statement 1: In p-type semiconductors, the drift velocity of charge carrier holes is higher than that of electrons,

Statement 2: In p-type semiconductors, the majority of charge carriers are holes.

Answer: 4. Statement 1 is false, and statement 2 is true

Question 4.

Statement 1: In the CE mode of a transistor, if the input signal is applied at the base, then the output signal is obtained at the collector.

Statement 2: In a transistor, most of the emitter current is transformed into the collector current.

Answer: 2. Statement 1 is true, and the statement is true; statement 2 is not a correct explanation for statement 1.

Question 5.

Statement 1: The frequency of the output signal from a feedback oscillator depends on its feedback ratio.

Statement 2: A feedback oscillator circuit is made in such a way that the closed-loop gain of the amplifier reaches an infinite value.

Answer: 4. Statement 1 is false, and statement 2 is true

Question 6.

Statement 1: Despite the increase in doping level, the conductivity of the semiconductor does not change.

Statement 2: By increasing the doping level in the semiconductor, the concentration of one type of charge carrier (electrons or holes) is increased and at the same time, the concentration of other charge carriers decreases.

Answer: 4. Statement 1 is false, and statement 2 is true

Question 7.

Statement 1: If the frequency of light below a certain minimum value is made incident on a photodiode, then current will flow through it.

Statement 2: If the energy of incident photon is less than a minimum value, then in a photodiode there is a possibility of recombination of electron-hole pairs.

Answer: 3. Statement 1 is true, and statement 2 is false

Semiconductors And Electronics Match The Columns

Question 1.

Answer: 1-B, 2-D, 3-A, 4-C

Question 2. Match the following two columns in case of different uses of a transistor

Answer: 1-B, 2-D, 3-C, 4-A

Question 3. In an extrinsic semiconductor, n, p are the concentration of electrons and holes, v_e, v_h are drift velocities and e μ_h are mobilities of electrons and holes respectively, e = charge of an electron

Answer: 1-D, 2-A, 3-B, 4-C

Question 4. A voltage regulator circuit is formed by a Zener diode of the rating 5V-0.25W. The maximum unregulated voltage of an external battery is 8V.To keep the Zener current at a safe limit, a resistance R is connected to the circuit. The terminal voltage of the load resistance in voltage-regulated conditions is 4.9V. Some quantities and their corresponding values are given in the following two columns.

Answer: 1-C, 2-B, 3-D, 4-A