JEE Physics Semiconductor Electronics MCQs Set 23

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Chapter-wise Objective Questions: Semiconductor Electronics

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Multiple Choice Questions

Question 1: The transfer ratio β of transistor is 50. The input resistance of a transistor when used in C.E. (Common Emitter) configuration is 1 kΩ. The peak value of the collector A.C current for an A.C input voltage of 0.01 V peak is
(a) 100 μA
(b) 0.01 mA
(c) 0.25 mA
(d) 500 μA
Show Answer & Explanation

Answer: (d) 500 μA

Explanation:
1. Ib = \( 0.01/1000 = 10 \muA \).
2. Ic = β Ib = \( 50 \times 10 = 500 \muA \).

Teacher's Note:
1. Recall the basic fact.

Question 2: Which of the following statements is/are correct? I. Pure Si doped with trivalent impurity gives a p-type semiconductor. II. Majority carriers in an n-type semiconductor are holes. III. Minority carriers in a p-type semiconductor are electrons. IV. The resistance of intrinsic semiconductor decreases with increase of temperature.
(a) I only
(b) I, III and IV
(c) I and IV
(d) II only
Show Answer & Explanation

Answer: (b) I, III and IV

Explanation:
1. In an n-type semiconductor the majority carriers are electrons, so II is false.

Teacher's Note:
1. Recall the basic fact.

Question 3: The average value of output direct current in a half wave rectifier is
(a) \( I_0/\pi \)
(b) \( I_0/2 \)
(c) \( \pi I_0/2 \)
(d) \( 2I_0/\pi \)
Show Answer & Explanation

Answer: (a) \( I_0/\pi \)

Explanation:
1. \( I_{dc} = I_{0}/\pi \) for a half wave rectifier.

Teacher's Note:
1. Recall the basic fact.

Question 4: The drift current in a p-n junction is from the
(a) n-side to the p-side
(b) p-side to the n-side
(c) n-side to the p-side if the junction is forward-biased and in the opposite direction if it is reverse biased
(d) p-side to the n-side if the junction is forward-biased and in the opposite direction if it is reverse biased
Show Answer & Explanation

Answer: (a) n-side to the p-side

Explanation:
1. Drift of minority carriers is caused by the junction field, from n-side to p-side.

Teacher's Note:
1. Recall the basic fact.

Question 5: A transistor has three impurity regions. All the three regions have different doping levels. In order of increasing doping level, the regions are
(a) emitter, base and collector
(b) collector, base and emitter
(c) base, emitter and collector
(d) base, collector and emitter
Show Answer & Explanation

Answer: (d) base, collector and emitter

Explanation:
1. Base is lightest, collector moderate and emitter heaviest.

Teacher's Note:
1. Recall the basic fact.

Question 6: A common emitter amplifier has a voltage gain of 50, an input impedance of 100 Ω and an output impedance of 200 Ω. The power gain of the amplifier is
(a) 500
(b) 1000
(c) 1250
(d) 50
Show Answer & Explanation

Answer: (c) 1250

Explanation:
1. Current gain = Av × Rin/Rout = \( 50 \times 100/200 = 25 \).
2. Power gain = \( 50 \times 25 = 1250 \).

Teacher's Note:
1. Recall the basic fact.

Question 7: For a junction diode the ratio of forward current (If) to reverse current (Ir) is [e = electronic charge, V = voltage applied across junction, k = Boltzmann constant, T = temperature in kelvin]
(a) \( e^{-V/kT} \)
(b) \( e^{V/kT} \)
(c) \( (e^{-eV/kT}+1) \)
(d) \( (e^{eV/kT}-1) \)
Show Answer & Explanation

Answer: (d) \( (e^{eV/kT}-1) \)

Explanation:
1. I = \( I_{0}(e^{eV/kT} - 1) \) in forward bias and I = \( I_{0} \) in reverse bias.
2. So If/Ir = \( e^{eV/kT} - 1 \).

Teacher's Note:
1. Recall the basic fact.

Question 8: A transistor is essentially
(a) a current operated device
(b) power driven device
(c) a voltage operated device
(d) resistance operated device
Show Answer & Explanation

Answer: (a) a current operated device

Explanation:
1. The collector current is controlled by the base current.

Teacher's Note:
1. Recall the basic fact.

Question 9: What is the voltage gain in a common emitter amplifier, where input resistance is 3 Ω and load resistance 24 Ω, β = 0.6?
(a) 8.4
(b) 4.8
(c) 2.4
(d) 480
Show Answer & Explanation

Answer: (b) 4.8

Explanation:
1. Av = β RL/Ri = \( 0.6 \times 24/3 = 4.8 \).

Teacher's Note:
1. Recall the basic fact.

Question 10: A transistor has a base current of 1 mA and emitter current 90 mA. The collector current will be
(a) 90 mA
(b) 1 mA
(c) 89 mA
(d) 91 mA
Show Answer & Explanation

Answer: (c) 89 mA

Explanation:
1. Ie = Ib + Ic, so Ic = \( 90 - 1 = 89 \) mA.

Teacher's Note:
1. Recall the basic fact.

Question 11: When p-n junction diode is forward biased then
(a) both the depletion region and barrier height are reduced
(b) the depletion region is widened and barrier height is reduced
(c) the depletion region is reduced and barrier height is increased
(d) Both the depletion region and barrier height are increased
Show Answer & Explanation

Answer: (a) both the depletion region and barrier height are reduced

Explanation:
1. Forward bias shrinks the depletion layer and lowers the barrier.

Teacher's Note:
1. Recall the basic fact.

Question 12: In reverse biasing
(a) large amount of current flows
(b) potential barrier across junction increases
(c) depletion layer resistance increases
(d) no current flows
Show Answer & Explanation

Answer: (b) potential barrier across junction increases

Explanation:
1. The applied voltage adds to the barrier, which widens the depletion region and raises the barrier.

Teacher's Note:
1. Recall the basic fact.

Question 13: On doping germanium with donor atoms of density 10¹⁷ cm⁻³, its conductivity in mho/cm will be [Given: μe = 3800 cm²/V·s and ni = 2.5 × 10¹³ cm⁻³]
(a) 30.4
(b) 60.8
(c) 91.2
(d) 121.6
Show Answer & Explanation

Answer: (b) 60.8

Explanation:
1. \( n \approx 10^{17} \) cm⁻³.
2. \( \sigma = n e \mue = 10^{17} \times 1.6 \times 10^{-19} \times 3800 = 60.8 \) mho/cm.

Teacher's Note:
1. Recall the basic fact.

Question 14: Amplifier may be
(a) multi stage
(b) single stage
(c) both (a) and (b)
(d) None of these
Show Answer & Explanation

Answer: (c) both (a) and (b)

Explanation:
1. Amplifiers may have one or several stages.

Teacher's Note:
1. Recall the basic fact.

Question 15: What is the value of A·C + A·B·C where A, B and C are inputs?
(a) \( A\cdot C \)
(b) \( A\cdot B \)
(c) \( A \)
(d) \( B \)
Show Answer & Explanation

Answer: (a) \( A\cdot C \)

Explanation:
1. \( A\cdot C + A\cdot B\cdot C = A\cdot C(1 + B) = A\cdot C \).

Teacher's Note:
1. Recall the basic fact.

Question 16: Current gain in common emitter configuration is more than 1 because
(a) \( I_c \lt I_b \)
(b) \( I_c \lt I_e \)
(c) \( I_c \gt I_e \)
(d) \( I_c \gt I_b \)
Show Answer & Explanation

Answer: (d) \( I_c \gt I_b \)

Explanation:
1. β = Ic/Ib and the collector current is much larger than the base current.

Teacher's Note:
1. Recall the basic fact.

Question 17:
Assertion (A): A p-n junction with reverse bias can be used as a photo-diode to measure light intensity.
Reason (R): In a reverse bias condition the current is small but it is more sensitive to changes in incident light intensity.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Light creates extra minority carriers, which changes the small reverse current by a large fraction, so the current measures intensity.

Teacher's Note:
1. Compare the two statements.

Question 18: To use a transistor as an amplifier, emitter-base junction is kept in X and base-collector junction is kept in Y. Here X and Y refer to
(a) forward bias, forward bias
(b) reverse bias, reverse bias
(c) reverse bias, forward bias
(d) forward bias, reverse bias
Show Answer & Explanation

Answer: (d) forward bias, reverse bias

Explanation:
1. For amplification the emitter junction is forward biased and the collector junction reverse biased.

Teacher's Note:
1. Recall the basic fact.

Question 19: In the half wave rectifier circuit operating from 50 Hz mains frequency, the fundamental frequency in the ripple would be
(a) 25 Hz
(b) 50 Hz
(c) 70.7 Hz
(d) 100 Hz
Show Answer & Explanation

Answer: (b) 50 Hz

Explanation:
1. A half wave rectifier gives one pulse per cycle, so the ripple frequency is 50 Hz.

Teacher's Note:
1. Recall the basic fact.

Question 20: In an insulator, the forbidden energy gap between the valence band and conduction band is of the order of
(a) 1 MeV
(b) 0.1 MeV
(c) 1 eV
(d) 5 eV
Show Answer & Explanation

Answer: (d) 5 eV

Explanation:
1. Insulators have a large gap, about 5 eV or more.

Teacher's Note:
1. Recall the basic fact.

Question 21: In a semiconductor
(a) there are no free electrons at 0 K
(b) there are no free electrons at any temperature
(c) the number of free electrons increases with pressure
(d) the number of free electrons is more than that in a conductor
Show Answer & Explanation

Answer: (a) there are no free electrons at 0 K

Explanation:
1. At \( 0 K \) all electrons are in the valence band, so a pure semiconductor has no free electrons.

Teacher's Note:
1. Recall the basic fact.

Question 22: For transistor action, which of the following statements are correct? I. Base, emitter and collector region have similar size and doping concentrations. II. The base must be very thin and lightly doped. III. The emitter junction is forward biased and collector junction is reverse biased. IV. Both emitter and collector junctions are forward biased.
(a) I and II
(b) II and III
(c) III and IV
(d) I and IV
Show Answer & Explanation

Answer: (b) II and III

Explanation:
1. The base is thin and lightly doped, and the emitter junction is forward biased while the collector junction is reverse biased.

Teacher's Note:
1. Recall the basic fact.

Question 23: Match the regions of a transistor in Column I with their description in Column II.
Column IColumn II
(A) Moderate size and heavily doped(1) Base
(B) Very thin and lightly doped(2) Collector
(C) Moderately doped and of large size(3) Emitter
(D) Doped with pentavalent impurity(4) N-type semiconductor

(a) A-3, B-1, C-2, D-4
(b) A-2, B-1, C-3, D-4
(c) A-4, B-1, C-2, D-3
(d) A-3, B-1, C-4, D-2
Show Answer & Explanation

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

Explanation:
1. The emitter is moderate in size and heavily doped.
2. The base is very thin and lightly doped.
3. The collector is large and moderately doped.
4. Pentavalent doping gives n-type material.

Teacher's Note:
1. Recall the structure of a transistor.

Question 24: Diffusion current in a p-n junction is greater than the drift current in magnitude
(a) if the junction is forward-biased
(b) if the junction is reverse-biased
(c) if the junction is unbiased
(d) in no case
Show Answer & Explanation

Answer: (a) if the junction is forward-biased

Explanation:
1. Forward bias lowers the barrier and increases diffusion current.

Teacher's Note:
1. Recall the basic fact.

Question 25: Temperature coefficient of resistance of semiconductor is
(a) zero
(b) constant
(c) positive
(d) negative
Show Answer & Explanation

Answer: (d) negative

Explanation:
1. Resistance falls as temperature rises, so the coefficient is negative.

Teacher's Note:
1. Recall the basic fact.

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Practice MCQs for JEE Physics Semiconductor Electronics

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