JEE Physics Semiconductor Electronics MCQs Set 24

Practice MCQs for JEE Physics Semiconductor Electronics

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Multiple Choice Questions

Question 1: In a common emitter (CE) amplifier having a voltage gain G, the transistor used has transconductance 0.03 mho and current gain 25. If the above transistor is replaced with another one with transconductance 0.02 mho and current gain 20, the voltage gain will be
(a) \( 1.5G \)
(b) \( \frac{1}{3}G \)
(c) \( \frac54G \)
(d) \( \frac23G \)
Show Answer & Explanation

Answer: (d) \( \frac23G \)

Explanation:
1. Av = β RL/Ri = gm RL, so Av is proportional to the transconductance.
2. New gain = \( (0.02/0.03)G = 2G/3 \).

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

Question 2: What is the conductivity of a semiconductor if electron density = 5 × 10¹²/cm³, hole density = 8 × 10¹³/cm³ (μe = 2.3 m² V⁻¹ s⁻¹, μh = 0.01 m² V⁻¹ s⁻¹)?
(a) 5.634 Ω⁻¹m⁻¹
(b) 1.968 Ω⁻¹m⁻¹
(c) 3.421 Ω⁻¹m⁻¹
(d) 8.964 Ω⁻¹m⁻¹
Show Answer & Explanation

Answer: (b) 1.968 Ω⁻¹m⁻¹

Explanation:
1. ne = \( 5 \times 10^{18} m^{-3} \) and nh = \( 8 \times 10^{19} m^{-3} \).
2. σ = e(neμe + nhμh) = \( 1.6 \times 10^{-19}(1.15 \times 10^{19} + 8 \times 10^{17}) = 1.968 \) Ω⁻¹m⁻¹.

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

Question 3: The gate for which output is high if at least one input is low is
(a) NAND
(b) NOR
(c) AND
(d) OR
Show Answer & Explanation

Answer: (a) NAND

Explanation:
1. NAND = NOT(A·B), which is 1 unless both inputs are 1.

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

Question 4: NAND and NOR gates are called universal gates primarily because they
(a) are available universally
(b) can be combined to produce OR, AND and NOT gates
(c) are widely used in Integrated circuit packages
(d) are easiest to manufacture
Show Answer & Explanation

Answer: (b) can be combined to produce OR, AND and NOT gates

Explanation:
1. Any logic function can be built from NAND or NOR gates alone.

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

Question 5: At absolute zero, Si acts as
(a) non-metal
(b) metal
(c) insulator
(d) None of these
Show Answer & Explanation

Answer: (c) insulator

Explanation:
1. With a completely filled valence band and an empty conduction band it behaves as an insulator.

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

Question 6: A p-type semiconductor is
(a) positively charged
(b) negatively charged
(c) uncharged
(d) uncharged at 0 K but charged at higher temperatures
Show Answer & Explanation

Answer: (c) uncharged

Explanation:
1. Doping adds atoms with their own electrons, so the crystal stays electrically neutral.

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

Question 7: In a semiconductor, the forbidden energy gap between the valence band and the conduction band is of the order of
(a) 1 MeV
(b) 0.1 MeV
(c) 1 eV
(d) 5 eV
Show Answer & Explanation

Answer: (c) 1 eV

Explanation:
1. The gap is about 1 eV (Si 1.1 eV, Ge 0.71 eV).

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

Question 8: By increasing the temperature, the specific resistance of a conductor and a semiconductor
(a) increases for both
(b) decreases for both
(c) increases, decreases
(d) decreases, increases
Show Answer & Explanation

Answer: (c) increases, decreases

Explanation:
1. The resistivity of a conductor increases with temperature and that of a semiconductor decreases.

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

Question 9: In a transistor
(a) the emitter has the least concentration of impurity
(b) the collector has the least concentration of impurity
(c) the base has the least concentration of impurity
(d) all the three regions have equal concentrations of impurity
Show Answer & Explanation

Answer: (c) the base has the least concentration of impurity

Explanation:
1. The base is thin and lightly doped, the emitter is heavily doped and the collector is moderately doped.

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

Question 10: In a n-type semiconductor, which of the following statements are incorrect? I. Electrons are minority carriers and pentavalent atoms are dopants. II. Holes are minority carriers and pentavalent atoms are dopants. III. Holes are majority carriers and trivalent atoms are dopants. IV. Electrons are majority carriers and trivalent atoms are dopants.
(a) I and II
(b) I, III and IV
(c) III and IV
(d) I, II and III
Show Answer & Explanation

Answer: (b) I, III and IV

Explanation:
1. In n-type, electrons are majority carriers and pentavalent atoms are the dopants.
2. So I, III and IV are incorrect and II is correct.

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

Question 11: Number of electrons in the valence shell of a pure semiconductor is
(a) 1
(b) 2
(c) 3
(d) 4
Show Answer & Explanation

Answer: (d) 4

Explanation:
1. Silicon and germanium are tetravalent.

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

Question 12: The current gain β may be defined as
(a) the ratio of change in collector current to the change in emitter current for a constant collector voltage in a common base arrangement
(b) the ratio of change in collector current to the change in the base current at constant collector voltage in a common emitter circuit
(c) the ratio of change in emitter current to the change in base current for constant emitter voltage in common emitter circuit
(d) the ratio of change in base current to the change in collector current at constant collector voltage in common emitter circuit
Show Answer & Explanation

Answer: (b) the ratio of change in collector current to the change in the base current at constant collector voltage in a common emitter circuit

Explanation:
1. β = (ΔIc/ΔIb) at constant Vce in the common emitter circuit.

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

Question 13: The truth table with (A, B) → C: (0,0) → 1, (0,1) → 1, (1,0) → 1, (1,1) → 0 is for which gate?
(a) XOR
(b) OR
(c) AND
(d) NAND
Show Answer & Explanation

Answer: (d) NAND

Explanation:
1. The output is 0 only when both inputs are 1: NAND.

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

Question 14:
Assertion (A): A transistor amplifier in common emitter configuration has a low input impedence.
Reason (R): The base to emitter region is forward biased.
(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. The forward biased emitter-base junction has a small dynamic resistance, which gives a low input impedance.

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

Question 15: What is the resistivity of a pure semiconductor at absolute zero?
(a) Zero
(b) Infinity
(c) Same as that of conductors at room temperature
(d) Same as that of insulators at room temperature
Show Answer & Explanation

Answer: (b) Infinity

Explanation:
1. At \( 0 K \) there are no free carriers, so conductivity is zero and resistivity is infinite.

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

Question 16:
Assertion (A): When two semiconductor of p and n type are brought in contact, they form p-n junction which act like a rectifier.
Reason (R): A rectifier is used to convert alternating current into direct current.
(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: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A \( p-n \) junction conducts only one way, so it rectifies.
2. The reason defines a rectifier but does not explain why the junction behaves as one.

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

Question 17: Which of these is/are correct? I. In forward biasing holes from p-side cross the junction and reach n-side. II. In forward biasing electrons from n-side cross the junction and reach p-side. III. In n-side holes are minority carriers. IV. In p-side electrons are minority carriers.
(a) I, II and III
(b) I, III and IV
(c) II, III and IV
(d) I, II, III and IV
Show Answer & Explanation

Answer: (d) I, II, III and IV

Explanation:
1. All four statements are correct.

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

Question 18: The intrinsic conductivity of germanium at 27°C is 2.13 mho m⁻¹ and mobilities of electrons and holes are 0.38 and 0.18 m²V⁻¹s⁻¹ respectively. The density of charge carriers is
(a) 2.37 × 10¹⁹ m⁻³
(b) 3.28 × 10¹⁹ m⁻³
(c) 7.83 × 10¹⁹ m⁻³
(d) 8.47 × 10¹⁹ m⁻³
Show Answer & Explanation

Answer: (a) 2.37 × 10¹⁹ m⁻³

Explanation:
1. \( \sigma = e \) ni(μe \( + \muh \)).
2. ni = \( 2.13/(1.6 \times 10^{-19} \times 0.56) = 2.38 \times 10^{19} m^{-3} \).

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

Question 19:
Assertion (A): Silicon is preferred over germanium for making semiconductor devices.
Reason (R): The energy gap in germanium is more than the energy gap in silicon.
(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: (c) A is true, but R is false.

Explanation:
1. Silicon has a larger gap, so it has a lower leakage current at normal temperature.
2. Germanium has the smaller gap, so the reason is false.

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

Question 20: A semiconductor device is connected in a series circuit with a battery and a resistance. A current is found to pass through the circuit. If the polarity of the battery is reversed, the current drops to almost zero. The device may be a/an
(a) intrinsic semiconductor
(b) p-type semiconductor
(c) n-type semiconductor
(d) p-n junction diode
Show Answer & Explanation

Answer: (d) p-n junction diode

Explanation:
1. Only a \( p-n \) junction conducts in one direction and blocks in the other.

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

Question 21: The relation between number of free electrons (n) in a semiconductor and temperature (T) is given by
(a) \( n\propto T \)
(b) \( n\propto T^2 \)
(c) \( n\propto\sqrt T \)
(d) \( n\propto T^{3/2} \)
Show Answer & Explanation

Answer: (d) \( n\propto T^{3/2} \)

Explanation:
1. \( n = A T^{3}/2) e^(-Eg/2kT \)), so \( n ∝ T^{3}/2 \)) apart from the exponential factor.

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

Question 22: Negative feedback
(a) increases stability
(b) decreases stability
(c) produces oscillation
(d) stops current in the tube
Show Answer & Explanation

Answer: (a) increases stability

Explanation:
1. Negative feedback stabilises gain and reduces distortion.

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

Question 23: If the forward bias on a p-n junction is increased from zero to 0.045 V, then no current flows in the circuit. The contact potential of junction i.e. VB is
(a) zero
(b) 0.045 V
(c) more than 0.045 V
(d) less than 0.045 V
Show Answer & Explanation

Answer: (c) more than 0.045 V

Explanation:
1. Current flows only when the applied forward bias exceeds the barrier, so VB > \( 0.045 V \).

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

Question 24: In the middle of the depletion layer of a reverse-biased p-n junction, the
(a) electric field is zero
(b) potential is maximum
(c) electric field is maximum
(d) potential is zero
Show Answer & Explanation

Answer: (c) electric field is maximum

Explanation:
1. The electric field of the double layer is greatest at the junction itself.

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

Question 25: In an npn transistor 10¹⁰ electrons enter the emitter in 10⁻⁶ s. 4% of the electrons are lost in the base. The current transfer ratio will be
(a) 0.98
(b) 0.97
(c) 0.96
(d) 0.94
Show Answer & Explanation

Answer: (c) 0.96

Explanation:
1. 96% of the emitter electrons reach the collector, so \( \alpha = 0.96 \).

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

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Practice MCQs for JEE Physics Semiconductor Electronics

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