Semiconductor Devices and Circuits

Semiconductor Devices and Circuits:

100 Multiple Choice Questions

Section A: Diodes and Physics

1. In a PN junction, the depletion region contains: A) Free electrons B) Free holes C) Immobile ions D) Neutral atoms
2. The width of the depletion layer of a PN junction diode ________ with increase in reverse bias. A) Decreases B) Increases C) Remains constant D) Becomes zero
3. The potential barrier of a Silicon diode at room temperature is approximately: A) 0.3 V B) 0.7 V C) 1.1 V D) 1.5 V
4. Zener breakdown occurs primarily due to: A) Impact ionization B) High doping concentration and strong electric field C) Thermal heating D) High reverse current
5. Avalanche breakdown is observed in: A) Lightly doped diodes B) Heavily doped diodes C) Forward biased diodes D) Zener diodes below 5V
6. A varactor diode acts as a variable: A) Resistor B) Inductor C) Capacitor D) Source
7. The main application of a Tunnel diode is in: A) Rectification B) High-speed switching and oscillators C) Audio amplification D) Voltage regulation
8. Which diode shows a negative resistance region in its V-I characteristics? A) Zener Diode B) LED C) Tunnel Diode D) Schottky Diode
9. In a solar cell, the open-circuit voltage is typically around: A) 0.1 V B) 0.5 to 0.6 V C) 2.0 V D) 12 V
10. The capacitance appearing across a reverse-biased semiconductor junction is called: A) Storage capacitance B) Diffusion capacitance C) Transition capacitance D) Plate capacitance
11. Diffusion capacitance is dominant in: A) Reverse bias region B) Forward bias region C) Zero bias region D) Breakdown region
12. An ideal diode acts as a ________ when forward biased and as a ________ when reverse biased. A) Open switch, Closed switch B) Closed switch, Open switch C) Resistor, Inductor D) Source, Sink
13. The dynamic resistance of a diode is proportional to: A) Current B) 1 / Current C) Current squared D) Voltage
14. For a Zener diode to work as a voltage regulator, it must be connected in: A) Forward bias B) Reverse bias C) Series with load D) Parallel with source
15. Schottky diodes are known for: A) High noise B) Slow switching C) Low forward voltage drop and fast switching D) High reverse breakdown voltage

Section B: Rectifiers and Filters

16. The maximum efficiency of a half-wave rectifier is: A) 40.6% B) 50% C) 81.2% D) 100%
17. The ripple factor of a full-wave rectifier is: A) 1.21 B) 0.48 C) 0.1 D) 2.0
18. In a bridge rectifier, the Peak Inverse Voltage (PIV) across each non-conducting diode is: A) $V_m$ B) $2V_m$ C) $V_m / 2$ D) $4V_m$
19. Which rectifier requires a center-tapped transformer? A) Half-wave B) Bridge C) Full-wave (two diode) D) None
20. A capacitor filter connected across a rectifier output: A) Increases ripple B) Decreases DC output voltage C) Increases DC output voltage and reduces ripple D) Has no effect
21. If the line frequency is 50 Hz, the ripple frequency in a full-wave rectifier is: A) 50 Hz B) 100 Hz C) 25 Hz D) 200 Hz
22. The DC output voltage of a bridge rectifier with peak input $V_m$ is: A) $V_m / \pi$ B) $2V_m / \pi$ C) $V_m / 2$ D) $V_m$
23. Which filter is most suitable for heavy loads (large currents)? A) Capacitor filter B) Inductor (Choke) filter C) RC filter D) None of these
24. A voltage doubler circuit uses: A) Diodes and Resistors B) Diodes and Inductors C) Diodes and Capacitors D) Transistors
25. Regulation is defined as the change in output voltage from: A) Zero load to Full load B) 10% load to 50% load C) Input to Output D) Low temp to High temp

Section C: Transistors (BJT)

26. In a BJT, the most heavily doped region is: A) Base B) Collector C) Emitter D) All are equal
27. The base of a transistor is made very thin to: A) Reduce cost B) Allow heat dissipation C) Reduce recombination of charge carriers D) Increase voltage rating
28. The relation between $\alpha$ and $\beta$ is: A) $\beta = \alpha / (1 – \alpha)$ B) $\beta = \alpha / (1 + \alpha)$ C) $\alpha = \beta / (1 – \beta)$ D) $\beta = 1 – \alpha$
29. If $\alpha = 0.98$, the value of $\beta$ is: A) 49 B) 98 C) 100 D) 0.02
30. In the active region of a transistor: A) E-B junction is reverse biased, C-B is forward biased B) E-B is forward biased, C-B is reverse biased C) Both junctions are forward biased D) Both junctions are reverse biased
31. A transistor acts as a closed switch in the: A) Active region B) Cutoff region C) Saturation region D) Inverse active region
32. The input impedance of a Common Collector (CC) configuration is: A) Very low B) Zero C) Medium D) Very high
33. Which configuration produces a phase shift of $180^\circ$ between input and output? A) Common Base B) Common Collector C) Common Emitter D) All of them
34. The leakage current $I_{CEO}$ is related to $I_{CBO}$ by: A) $I_{CEO} = I_{CBO}$ B) $I_{CEO} = (1+\beta) I_{CBO}$ C) $I_{CEO} = \beta I_{CBO}$ D) $I_{CEO} = I_{CBO} / (1+\beta)$
35. Ideally, the stability factor $S$ should be: A) As high as possible B) As low as possible (close to 1) C) Equal to $\beta$ D) Zero
36. Thermal runaway in a transistor occurs due to increase in: A) Base current B) Collector reverse leakage current C) Emitter resistance D) Supply voltage
37. The voltage divider bias is also known as: A) Fixed bias B) Base resistor bias C) Self bias (Universal bias) D) Feedback bias
38. The Early Effect in a BJT refers to: A) Base width modulation by collector voltage B) Heating of the collector C) Breakdown of the emitter D) Increase in $\beta$ with temperature
39. For amplification, the BJT is operated in: A) Saturation B) Cutoff C) Active region D) Breakdown
40. In a PNP transistor, the majority carriers in the emitter are: A) Electrons B) Holes C) Both equal D) Ions
41. A Common Base amplifier has a current gain ($\alpha$) of: A) Much greater than 1 B) Equal to $\beta$ C) Less than 1 D) Zero
42. Which amplifier configuration is used for impedance matching? A) Common Emitter B) Common Base C) Common Collector (Emitter Follower) D) Differential Amplifier
43. The Q-point in a transistor amplifier represents: A) The cutoff point B) The saturation point C) The DC operating point D) The AC maximum gain
44. The crossover distortion occurs in: A) Class A amplifiers B) Class B push-pull amplifiers C) Class C amplifiers D) Common Base amplifiers
45. The maximum theoretical efficiency of a Class B amplifier is: A) 25% B) 50% C) 78.5% D) 100%
46. Which transistor configuration has the highest power gain? A) CB B) CC C) CE D) All equal
47. In a fixed bias circuit, the operating point shifts due to changes in: A) Temperature and $\beta$ B) Capacitor values C) Input frequency D) Load resistance
48. The bypass capacitor in a CE amplifier is used to: A) Block DC B) Bypass the emitter resistor to prevent AC gain reduction C) Couple stages D) Filter noise
49. An emitter follower has a voltage gain of: A) > 100 B) Equal to $\beta$ C) Approximately 1 (slightly less) D) Infinite
50. The output characteristics of a CE transistor is a plot between: A) $I_B$ and $V_{BE}$ B) $I_C$ and $V_{CE}$ C) $I_E$ and $V_{CB}$ D) $I_C$ and $I_B$

Section D: Field Effect Transistors (FET)

51. A JFET is a ________ controlled device. A) Current B) Voltage C) Power D) Resistance
52. The input impedance of a JFET is: A) Very low B) Very high ($> 1 M\Omega$) C) Zero D) Same as BJT
53. The gate-source junction of a JFET is always: A) Forward biased B) Reverse biased C) Unbiased D) Grounded
54. The Pinch-off voltage ($V_P$) in a JFET is the voltage at which: A) Drain current becomes zero B) Drain current saturates C) Gate current becomes maximum D) Breakdown occurs
55. In an N-channel JFET, the carriers are: A) Holes B) Electrons C) Ions D) Both A and B
56. MOSFET stands for: A) Metal Oxide Silicon Field Effect Transistor B) Metal Oxide Semiconductor Field Effect Transistor C) Metal Organic Semiconductor Field Effect Transistor D) Metallic Oxide Source FET
57. The insulating layer in a MOSFET is usually made of: A) Silicon B) Silicon Dioxide ($SiO_2$) C) Germanium D) Copper
58. Which MOSFET can operate in both depletion and enhancement modes? A) D-MOSFET B) E-MOSFET C) JFET D) UJT
59. The transconductance ($g_m$) of a FET is defined as: A) $\Delta I_D / \Delta V_{GS}$ B) $\Delta V_{GS} / \Delta I_D$ C) $\Delta I_D / \Delta V_{DS}$ D) $\Delta V_{DS} / \Delta I_D$
60. E-MOSFETs are normally ________ devices. A) ON B) OFF C) Shorted D) Conducting
61. CMOS technology uses: A) Only NMOS B) Only PMOS C) Both NMOS and PMOS D) BJTs and FETs
62. The channel in a JFET is controlled by: A) Depletion region width B) Oxide thickness C) Doping level only D) Thermal agitation
63. At pinch-off, the channel width is: A) Maximum B) Minimum (effectively closed) C) Zero D) Unchanged
64. FETs have ________ thermal stability compared to BJTs. A) Better B) Worse C) Equal D) No
65. The relation between drain current and gate voltage in a JFET is: A) Linear B) Quadratic (Square law) C) Exponential D) Logarithmic

Section E: Amplifiers

66. An RC coupled amplifier is widely used for: A) DC amplification B) Voltage amplification in audio range C) RF amplification D) Power amplification
67. The fall in gain at low frequencies in an RC coupled amplifier is due to: A) Transistor beta B) Coupling and Bypass capacitors C) Shunt capacitance D) Wiring inductance
68. The fall in gain at high frequencies is due to: A) Coupling capacitors B) Inter-electrode and stray shunt capacitances C) Resistors D) Power supply
69. The bandwidth of an amplifier is the range of frequencies where gain is at least: A) 50% of max B) 70.7% ($-3$ dB) of max C) 90% of max D) 100% of max
70. Class A amplifier conduction angle is: A) $90^\circ$ B) $180^\circ$ C) $270^\circ$ D) $360^\circ$
71. Transformer coupling is used primarily for: A) Impedance matching and power transfer B) Voltage gain C) Reducing cost D) High frequency response
72. In a multistage amplifier, the total gain in dB is the ________ of individual gains in dB. A) Product B) Sum C) Difference D) Ratio
73. The Miller Effect results in an increase in: A) Input capacitance B) Output resistance C) Voltage gain D) Bandwidth
74. A Darlington pair connects two transistors to achieve: A) Very high voltage gain B) Very high current gain and input impedance C) Low input impedance D) High frequency operation
75. Distortion caused by non-linear characteristics of the device is called: A) Frequency distortion B) Phase distortion C) Harmonic (Amplitude) distortion D) Noise
76. If the gain is 100 and bandwidth is 10 kHz, the Gain-Bandwidth Product (GBW) is: A) $10^5$ Hz B) $10^6$ Hz C) $10^4$ Hz D) 100 Hz
77. Class C amplifiers are used in: A) Audio amplifiers B) RF Tuned amplifiers C) Oscillators D) Linear power supplies
78. The efficiency of a Class A amplifier with resistive load is maximum: A) 25% B) 50% C) 78.5% D) 90%
79. Noise in an amplifier is least in: A) Carbon resistors B) Wire wound resistors C) FETs D) Metal film resistors
80. A differential amplifier amplifies: A) The sum of input signals B) The difference of input signals C) Only one signal D) Common mode signals

Section F: Feedback and Oscillators

81. Negative feedback in an amplifier: A) Increases gain B) Reduces bandwidth C) Increases stability and bandwidth D) Increases distortion
82. The gain of an amplifier with feedback ($A_f$) is given by: A) $A / (1 – A\beta)$ B) $A / (1 + A\beta)$ C) $A(1 + A\beta)$ D) $1 / \beta$
83. The product $A\beta$ is called: A) Closed loop gain B) Open loop gain C) Loop gain D) Feedback factor
84. Emitter follower uses ________ feedback. A) Voltage Series Negative B) Current Series Negative C) Voltage Shunt Negative D) Positive
85. Positive feedback is necessary for: A) Stabilization B) Oscillation C) Noise reduction D) Bandwidth extension
86. The Barkhausen criterion for oscillation is: A) $|A\beta| = 1$ and phase shift $0^\circ$ or $360^\circ$ B) $|A\beta| < 1$ C) Phase shift $180^\circ$ D) $|A\beta| = 0$
87. Which oscillator uses a split capacitor tank circuit? A) Hartley B) Colpitts C) RC Phase Shift D) Wien Bridge
88. A Hartley oscillator uses: A) Tapped inductor B) Tapped capacitor C) RC network D) Crystal
89. An RC Phase Shift oscillator generally uses: A) 1 RC section B) 2 RC sections C) 3 RC sections D) 4 RC sections
90. The most stable oscillator for frequency is: A) Hartley B) Colpitts C) Crystal Oscillator D) Wien Bridge
91. In a Wien Bridge oscillator, the feedback network provides zero phase shift at frequency: A) $1 / (2\pi RC)$ B) $1 / (2\pi\sqrt{LC})$ C) $1 / (4\pi RC)$ D) $RC$
92. Negative feedback ________ input impedance in voltage-series feedback. A) Decreases B) Increases C) Keeps constant D) Makes zero
93. A multivibrator is a type of: A) Sinusoidal oscillator B) Relaxation oscillator (Square wave generator) C) Amplifier D) Rectifier
94. The feedback fraction $\beta$ is: A) Output / Input B) Feedback Voltage / Output Voltage C) Input / Output D) Gain / Input
95. If $A = 100$ and $\beta = 0.09$, the gain with negative feedback is approx: A) 90 B) 100 C) 10 D) 9
96. Crystal oscillators work on the principle of: A) Photoelectric effect B) Piezoelectric effect C) Compton effect D) Hall effect
97. Current-shunt negative feedback behaves as a: A) Voltage amplifier B) Current amplifier C) Transresistance amplifier D) Transconductance amplifier
98. To start oscillations, initially $|A\beta|$ must be: A) Exactly 1 B) Slightly greater than 1 C) Less than 1 D) Zero
99. An astable multivibrator has: A) One stable state B) Two stable states C) No stable states D) Three stable states
100. The active element in a Phase Shift oscillator is typically: A) Diode B) Transformer C) Transistor or Op-Amp D) Inductor

Part 3: Answer Key and Detailed Solutions

1. C) The depletion region is formed by the recombination of electrons and holes, leaving behind immobile positive and negative ions. 2. B) Reverse bias pulls carriers away from the junction, widening the depletion layer. 3. B) For Silicon, the barrier potential is $\approx 0.7V$; for Germanium, it is $\approx 0.3V$. 4. B) Zener breakdown happens in heavily doped diodes where the narrow depletion region creates a strong electric field, pulling electrons from bonds. 5. A) Avalanche breakdown occurs in lightly doped diodes due to carrier collision/impact ionization at high voltages. 6. C) A reverse-biased diode acts as a capacitor where the depletion width (dielectric) varies with voltage. 7. B) Tunnel diodes exhibit negative resistance, making them useful for high-speed switching and microwave oscillators. 8. C) Due to quantum tunneling, current decreases as voltage increases in a specific range. 9. B) A single silicon solar cell typically produces about 0.5V to 0.6V DC. 10. C) Transition (or depletion) capacitance dominates in reverse bias due to charge separation by the depletion layer. 11. B) Diffusion capacitance dominates in forward bias due to the storage of minority carriers near the junction. 12. B) Ideally, it’s a perfect conductor (switch closed) in forward bias and a perfect insulator (switch open) in reverse bias. 13. B) Dynamic resistance $r_d = \eta V_T / I$. It is inversely proportional to the DC bias current $I$. 14. B) Zener diodes are designed to work in the reverse breakdown region to maintain constant voltage. 15. C) Schottky diodes use a metal-semiconductor junction, resulting in no minority carrier storage, low voltage drop, and fast switching. 16. A) The theoretical maximum efficiency is $40.6\%$. 17. B) Full-wave rectifiers have a lower AC component (ripple) with $\gamma = 0.48$. 18. A) In a bridge, two diodes conduct at a time, so the reverse voltage across the OFF diodes is shared, effectively $V_m$ (unlike $2V_m$ in center-tapped). 19. C) A standard full-wave rectifier uses a center-tapped transformer; a bridge does not. 20. C) A capacitor charges to the peak voltage and discharges slowly, raising the average DC and smoothing the waveform (reducing ripple). 21. B) For full-wave, the output pulses twice per input cycle. $2 \times 50 = 100$ Hz. 22. B) $V_{dc} = 2V_m / \pi \approx 0.636 V_m$. 23. B) An inductor opposes current change. At heavy loads (high current), it filters effectively. Capacitor filters drop voltage at heavy loads. 24. C) Voltage doublers use a network of diodes and capacitors to clamp and stack voltages. 25. A) Regulation $\% = (V_{no-load} – V_{full-load}) / V_{full-load} \times 100$. 26. C) The Emitter is heavily doped to inject a large number of carriers into the base. 27. C) A thin base ensures that most carriers injected from the emitter pass through to the collector without recombining in the base. 28. A) $\beta = I_C/I_B$ and $\alpha = I_C/I_E$. Algebra yields $\beta = \alpha / (1-\alpha)$. 29. A) $\beta = 0.98 / (1 – 0.98) = 0.98 / 0.02 = 49$. 30. B) Active region (Amplification): Emitter-Base forward biased, Collector-Base reverse biased. 31. C) Saturation region: Both junctions forward biased, acting like a short (closed switch). 32. D) CC (Emitter Follower) has high input impedance and low output impedance. 33. C) Common Emitter configuration inverts the signal ($180^\circ$ shift). 34. B) $I_{CEO}$ is the leakage current from Collector to Emitter with Base open. It is amplified by the transistor: $(1+\beta)I_{CBO}$. 35. B) Stability factor $S$ measures change in $I_C$. Lower $S$ (ideally 1) means better stability against temperature changes. 36. B) Leakage current increases with temperature, increasing $I_C$, which increases heat, leading to a cycle (runaway). 37. C) It makes the operating point independent of $\beta$, hence “Universal”. 38. A) Higher $V_{CB}$ widens the collector depletion region, narrowing the effective base width (Base Width Modulation). 39. C) The active region provides linear amplification. 40. B) In PNP, P-type emitter injects Holes. 41. C) $I_C < I_E$, so $\alpha < 1$. 42. C) CC (Emitter Follower) matches high Z source to low Z load. 43. C) Quiescent point: The DC voltage and current values when no signal is applied. 44. B) Occurs when the transistor is switching on/off at the zero crossing (dead zone) in Push-Pull Class B. 45. C) $\pi / 4 \approx 78.5\%$. 46. C) CE has both voltage and current gain, giving the highest Power Gain. 47. A) Fixed bias is very unstable with respect to temperature and $\beta$ variations. 48. B) Without it, the emitter resistor provides negative feedback, reducing the AC gain. The capacitor shorts AC to ground. 49. C) Gain is slightly less than 1, but usually treated as unity. 50. B) Plot of $I_C$ vs $V_{CE}$ for constant $I_B$. 51. B) The Gate voltage controls the electric field which controls the current. 52. B) The gate is reverse biased, drawing almost zero current. 53. B) To maintain high input impedance and control depletion width. 54. B) $V_P$ is the voltage where the channel is fully depleted (pinched), and current saturates (becomes constant). 55. B) N-channel uses electrons (faster mobility than holes). 56. B) Metal Oxide Semiconductor Field Effect Transistor. 57. B) A silicon dioxide layer insulates the gate from the channel. 58. A) Depletion MOSFET (D-MOSFET) works with zero bias (channel exists) and can be depleted or enhanced. 59. A) Transconductance is the ratio of change in output current to change in input voltage. 60. B) E-MOSFETs have no physical channel at 0V. They need a gate voltage to turn ON. 61. C) Complementary MOS uses paired NMOS and PMOS for low power. 62. A) Reverse bias increases depletion width, narrowing the conductive channel. 63. B) It is not physically zero (current still flows), but it is minimum/constricted. 64. A) FETs have a negative temperature coefficient (resistivity increases with temp), preventing thermal runaway. 65. B) Shockley’s equation: $I_D = I_{DSS}(1 – V_{GS}/V_P)^2$. 66. B) Voltage amplification for audio signals. 67. B) Capacitors have high reactance ($X_C = 1/2\pi fC$) at low frequencies, blocking the signal. 68. B) Shunt (parasitic) capacitances short the signal to ground at high frequencies. 69. B) $1/\sqrt{2}$ of voltage gain corresponds to -3dB power point. 70. D) Conducts for the full cycle ($360^\circ$). 71. A) Transformers transform impedance ($n^2$) for max power transfer. 72. B) Gains in decibels are additive. $20dB + 10dB = 30dB$. 73. A) Miller effect multiplies the feedback capacitance by $(1+A)$, increasing input capacitance and reducing HF gain. 74. B) Cascading two transistors provides extremely high $\beta$ ($\beta_1 \times \beta_2$) and high input Z. 75. C) Introduction of frequencies (harmonics) not present in the input. 76. B) $100 \times 10^4 = 10^6$ Hz. 77. B) Conducts for $< 180^\circ$. High efficiency, used with tuned LC circuits in RF. 78. A) With a resistive load, max efficiency is 25%. With transformer load, 50%. 79. D) Metal film resistors have lower thermal noise than carbon. 80. B) It amplifies the difference ($V_1 – V_2$) and rejects common signals (noise). 81. C) Negative feedback trades gain for stability, linearity, and bandwidth. 82. B) The standard feedback formula. 83. C) Loop gain determines the stability margins. 84. A) Voltage is sampled (parallel/shunt output) and mixed in series (opposing $V_{in}$ in loop). Correction: Emitter Follower is Voltage-Series feedback. Output is Voltage (Emitter), Feedback is proportional to V_out, applied in Series with input. 85. B) Positive feedback (regeneration) sustains oscillations. 86. A) Loop gain magnitude must be 1, and phase shift around the loop must be $0^\circ$ or $360^\circ$ (positive feedback). 87. B) Colpitts uses a tapped capacitor divider. 88. A) Hartley uses a tapped inductor. 89. C) Each RC section provides up to $90^\circ$ (practically $60^\circ$). 3 sections provide $180^\circ$ total shift. 90. C) Crystals have very high Q-factor, providing precise frequency stability. 91. A) Frequency of oscillation for Wien Bridge. 92. B) Series mixing increases input impedance ($Z_{in}(1+A\beta)$). 93. B) It generates non-sinusoidal waves (square waves) by switching states. 94. B) $\beta = V_f / V_{out}$. It represents the fraction of output fed back. 95. C) $A_f = 100 / (1 + 100\times 0.09) = 100 / 10 = 10$. 96. B) Mechanical stress creates voltage and vice-versa. 97. B) Current sampling (increases $R_{out}$) and shunt mixing (decreases $R_{in}$) stabilizes current gain. 98. B) Slightly $>1$ to overcome losses and build up amplitude from noise. 99. C) Astable has no stable states; it continuously switches (oscillates). 100. C) The RC network provides phase shift, the active device (Transistor/Op-Amp) provides gain to overcome attenuation.