Semiconductor Devices and Communication Systems
Each of the following questions comprises two statements. The assertion is expressed by Statement 1 and the reason is expressed by Statement 2.
Each question has four options (1), (2), (3), and (4) as given below, out of which only ONE is correct.
- Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
- Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
- Statement 1 is true and Statement 2 is false.
- Statement 1 is false and Statement 2 is true.
- Both Statement 1 and Statement 2 are false.
Some questions are preceded, by a supporting paragraph in addition to the statements.
Semiconductors Transistors Logic Gates
Question 1. Statement 1 The resistance of a semiconductor decreases with a rise in temperature.
Statement 2 The energy gap between the conduction band and the valence band is very small.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The energy gap between the conduction band and the valence band in a semiconductor is very small. When the temperature is increased, electrons get sufficient thermal energy to jump over to the conduction band and participate in electrical conduction. Thus, an increase in temperature increases the current, thereby reducing the resistance.
Question 2. Statement 1 The resistivity of a semiconductor increases with temperature.
Statement 2 The atoms of a semiconductor vibrate with a large amplitude at higher temperatures, thereby increasing resistivity.
Answer: 5. Both Statement 1 and Statement 2 are false.
The resistivity of a semiconductor decreases with a rise in temperature due to the transfer of electrons from the valence band to the conduction band. This increases the number of electron-hole pairs.
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Question 3. Statement 1 An n-type semiconductor has a large number of electrons, but still, it is electrically neutral.
Statement 2 An n-type semiconductor is obtained by doping an intrinsic semiconductor with a pentavalent impurity.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
The doping of germanium (tetravalent) with a pentavalent impurity (phosphorus, arsenic, or antimony) makes an n-type semiconductor. Since the atom on the whole is electrically neutral, the extrinsic (n-type or p-type) semiconductor is also neutral.
Question 4. Statement 1 The energy gap between the valence band and the conduction band is greater in silicon than in germanium.
Statement 2 Thermal energy produces fewer minority carriers in silicon than in germanium.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The energy-band gap in silicon is 1.14 eV and that in germanium is 0.67 eV. In silicon atoms, electrons are more tightly bound to the nucleus, so there is more energy gap.
Question 5. Statement 1 The number of electrons in a p-type silicon semiconductor is less than that in a pure silicon semiconductor at room temperature.
Statement 2 It is due to the law of mass action.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
According to the law of mass action,
∴ \(n_{\mathrm{i}}^2=n_{\mathrm{e}} n_{\mathrm{h}}\)
In a p-type semiconductor, nh>ne.
Question 6. Statement 1 The temperature coefficient of resistance is positive for metals and negative for p-type semiconductors.
Statement 2 The effective charge carriers in metals are negatively charged, whereas they are positively charged in p-type semiconductors.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
The resistance of a metallic conductor increases with temperature due to an increase in the amplitude of vibrations of atoms. This obstructs the electron flow. Thus, metals have a positive temperature coefficient of resistance.
In metals, the charge carriers are electrons, whereas, in p-type semiconductors, the majority of carriers are holes.
Question 7. Statement 1 Insulators do not allow the flow of current through themselves.
Statement 2 They have no free-charge carriers.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
In an insulator, the conduction band is completely empty and the valence band is completely filled. Further, the energy gap (Eg)is greater than kT. So, the thermal energy is insufficient for the electrons to jump over to the conduction band.
Question 8. Statement 1 A p-n junction with a reverse bias can be used as a photodiode to measure the intensity of light.
Statement 2 In a reverse-bias condition, the current is small but more sensitive to a change in the intensity of the incident light.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
A photodiode converts light into electrical energy. It is a p-n junction operated in the reverse bias. When the device is exposed to illumination, the reverse current increases linearly with the intensity of the incident light.
Question 9. Statement 1 During reverse-biasing, a diode does not conduct current.
Statement 2 It decreases the thickness of the depletion layer.
Answer: 3. Statement 1 is true and Statement 2 is false.
A p-n junction in the reverse bias offers a large resistance to the current. The thickness of the depletion layer widens.
Question 10. Statement 1 A pure semiconductor has a negative temperature coefficient of resistance.
Statement 2 On raising the temperature, more charge carriers are released. This increases the conductance and decreases the resistance of a semiconductor.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
When the temperature of an intrinsic (pure) semiconductor is raised, more electrons acquire sufficient thermal energy to jump over to the conduction band, so the conductivity increases and the resistivity decreases. This corresponds to a negative temperature coefficient of resistance.
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Question 11. Statement 1 At a fixed temperature, silicon will have a minimum conductivity when it has a smaller acceptor doping.
Statement 2 The conductivity of an intrinsic semiconductor is slightly higher than that of a lightly doped p-type semiconductor.
Answer: 3. Statement 1 is true and Statement 2 is false.
The conductivity of an intrinsic semiconductor (pure silicon) is less than that of a semiconductor lightly doped with a p- or n-type impurity.
Question 12. Statement 1 A photocell is called an electric eye.
Statement 2 When light is incident on some semiconductor, its electrical resistance is reduced.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
A photoresistor, or light-dependent resistor (LDR), is a variable resistor whose resistance decreases with increasing intensity of the incident light. Thus, it exhibits photoconductivity. These are used as sensors to detect light and are hence called electric eyes.
Question 13. Statement 1 Diode lasers are used as optical sources in optical communications.
Statement 2 Diode lasers consume less energy.
Answer: 2. Statement 1 is true; Statement 2 is true; but Statement 2 is not a correct explanation of Statement 1.
A diode laser uses semiconductor technology that produces coherent light between the visible and infrared ranges. These are used in optical communications for high-speed data transmissions. They are efficient in converting power into optical power with a minimum loss of energy.
Question 14. Statement 1 A photodiode and a photovoltaic cell are both based on the same principle.
Statement 2 Both use the same method of operation to work.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
Both a photodiode and a photovoltaic (solar) cell generate emfs and convert light into electrical energy when solar radiations fall on its p-n junction.
Question 15. Statement 1 In a common-emitter (CE) amplifier, the load resistance of the output circuit is 1000 times the load resistance of the input circuit. If α = 0.98 then the voltage gain is 49 x 103.
Statement 2 \(\alpha=\frac{\beta}{1-\beta}\), where the symbols have their usual meanings.
Answer: 3. Statement 1 is true and Statement 2 is false.
⇒ \(I_{\mathrm{e}}=I_{\mathrm{c}}+I_{\mathrm{b}} \Rightarrow \frac{I_{\mathrm{e}}}{I_{\mathrm{c}}}=1+\frac{I_{\mathrm{b}}}{I_{\mathrm{c}}}\)
⇒ \(\frac{1}{\alpha}=1+\frac{1}{\beta} \Rightarrow \alpha=\frac{\beta}{1+\beta}\)
Voltage gain = \(\frac{V_{\mathrm{o}}}{V_{\mathrm{i}}}=\frac{R_{\mathrm{o}} I_{\mathrm{c}}}{R_{\mathrm{i}} I_{\mathrm{b}}}=\beta\left(\frac{R_{\mathrm{o}}}{R_{\mathrm{i}}}\right)\)
∴ \(\frac{\alpha}{1-\alpha}\left(\frac{R_{\mathrm{o}}}{R_{\mathrm{i}}}\right)=\frac{0.98}{0.02} \times 1000=49 \times 10^3\)
Question 16. Statement 1 In a transistor, the base is made thin.
Statement 2 A thin base makes a transistor stable.
Answer: 3. Statement 1 is true and Statement 2 is false.
In a transistor, the base is very thin and lightly doped, so that very few holes combine with electrons in the base region and most of the holes diffuse through the base to the collector region.
Question 17. Statement 1 Most amplifiers use the common-emitter (CE) circuit configuration.
Statement 2 The input resistance of the CE amplifier is comparatively higher.
Answer: 3. Statement 1 is true and Statement 2 is false.
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Transistor amplifiers in the common-emitter (CE) mode are mostly preferred due to the following features:
- High current gain,
- High voltage gain,
- Low input resistance,
- High output resistance.
Question 18. Statement 1 In a common-emitter transistor amplifier, the input current is much less than the output current.
Statement 2 A common-emitter transistor amplifier has a very high input impedance.
Answer: 3. Statement 1 is true and Statement 2 is false.
A transistor amplifier in the CE mode has a high current gain. Hence, the input current is much less than the output current and has a low input impedance.
Question 19. Statement 1 A transistor amplifier in the CE mode has a low input impedance.
Statement 2 The base-emitter region is forward-biased.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
A transistor in the CE configuration has its input junction (emitter-base junction) forward-biased. So, it has a low input impedance.
Question 20. Statement 1 In the common-base (CB) configuration, the current gain of the transistor is less than unity.
Statement 2 The collector terminal is reverse-biased for amplification.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The current gain in the common-base mode of a transistor is very small because the output region (collector-base side) is reverse-biased for amplification.
Question 21. Statement 1 NAND is a universal gate.
Statement 2 It can be used to describe all other logic gates.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
NAND and NOR gates are called universal gates, as all other gates can be obtained by using them.
Question 22. Statement 1 A NOT gate can be built using a diode.
Statement 2 The output voltage and the input voltage of the diode have a 180° phase difference.
Answer: 5. Both Statement 1 and Statement 2 are false.
A NOT logic gate can be built not by a p-n junction diode but by an n-p-n transistor in which the output is in the opposite phase with the input.
Communication Systems
Question 1. Statement 1 In a communication system based on amplitude modulation, the modulation index is kept less than 1.
Statement 2 It ensures the minimum distortion of signals.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The modulation index is defined as the ratio of the amplitude of the modulating signal (Am) to that of the carrier wave (Ac). If the modulation index (μ = Am/Ac) is greater than 1, this will result in a distortion due to overmodulation. Thus, for the minimum distortion, μ<1.
Question 2. Statement 1 In the transmission of long-distance radio signals, the short-wave band is used.
Statement 2 For shorter wavelengths, attenuation is very low.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
In long-distance transmissions, the short-waveband is used, so that the dissipation (attenuation) of energy is minimum.
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Question 3. Statement 1 Television signals are received through sky-wave propagations.
Statement 2 The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.
Answer: 5. Both Statement 1 and Statement 2 are false.
Signals of frequencies more than 40 MHz are not reflected back to the earth. So, they cannot be used for long-distance transmissions. In TV transmissions, the frequency range used is much higher (≈ 100-200 MHz), and they do not get reflected.
Question 4. Statement 1 Short-wave communications over long distances are not possible via ground waves.
Statement 2 Ground waves can bend around the corners of the objects on the earth.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
Ground-wave propagations are useful only at low frequencies (not short waves) because the absorption of the waves increases considerably with frequency.
Question 5. Statement 1 Communications in the UHF and VHF regions can be established by space waves or tropospheric waves.
Statement 2 Communications in the UHF and VHF regions are limited to the line-of-sight distances.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
The UHF/VHF frequency range is not reflected by the ionosphere. So, they are limited to the direct, or LoS (line-of-sight), path.
Question 6. Statement 1 Sky-wave signals are used for long-distance radio communications. These signals are in general less stable than ground-wave signals.
Statement 2 The state of the ionosphere varies from hour to hour, from day to day, and from season to season.
Answer: 1. Statement 1 is true; Statement 2 is true; and Statement 2 is a correct explanation of Statement 1.
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Sky-wave transmissions are applicable for the frequency range (3-30 MHz). These radio waves can propagate through the atmosphere and are reflected by the ionosphere.
Due to the variation of properties of the ionospheric layers from time to time, sky waves are less stable compared to ground waves.
Question 7. Statement 1 Electromagnetic waves with frequencies smaller than the critical frequencies of the ionosphere cannot be used for communications using sky-wave propagations.
Statement 2 The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.
Answer: 5. Both Statement 1 and Statement 2 are false.
The critical frequency is the highest frequency above which electromagnetic waves get transmitted through the ionosphere and below which the waves are reflected from the ionosphere. The critical frequency is given by \(f_{\mathrm{c}}=9 \sqrt{N_{\max }}\), where Nmax is the electron density in the ionosphere. With frequencies greater than fc, there are transmissions with a small refractive index.