BEE Semiconductor Devices and Circuits 2 — Questions and Answers
Question 1: In a MOSFET, the threshold voltage VT is most directly affected by which parameter?
- Gate oxide thickness (Correct answer)
- Drain-source voltage
- Channel length modulation
- Minority carrier lifetime
Correct answer: Gate oxide thickness
Threshold voltage is directly related to oxide capacitance, which depends on gate oxide thickness (VT increases as tox increases).
Question 2: A zener diode is operated in reverse breakdown with VZ = 6.2 V and IZ = 20 mA. What power must it dissipate?
- 124 mW (Correct answer)
- 62 mW
- 310 mW
- 248 mW
Correct answer: 124 mW
Power = VZ × IZ = 6.2 V × 20 mA = 124 mW.
Question 3: Which biasing configuration gives a BJT the highest input impedance?
- Common base
- Common emitter
- Common collector (Correct answer)
- Differential pair
Correct answer: Common collector
The common-collector (emitter-follower) configuration exhibits the highest input impedance among standard BJT configurations.
Question 4: What is the primary function of the depletion region in a p-n junction?
- It stores minority carriers for conduction
- It creates a built-in electric field that opposes further diffusion (Correct answer)
- It increases the conductivity of the junction
- It provides the path for majority carrier current
Correct answer: It creates a built-in electric field that opposes further diffusion
The depletion region's ionized dopant atoms create a built-in electric field that counters carrier diffusion, establishing equilibrium.
Question 5: In the early effect (base-width modulation) of a BJT, increasing VCE causes the collector current to:
- Decrease due to increased recombination
- Remain constant regardless of VCE
- Increase slightly due to narrowing of the base (Correct answer)
- Decrease due to widening of the emitter-base junction
Correct answer: Increase slightly due to narrowing of the base
Increasing VCE widens the collector-base depletion region, narrowing the base and increasing the collector current gradient.
Question 6: An NMOS transistor has kn = 0.5 mA/V² and VTN = 1 V. With VGS = 3 V and VDS = 4 V, the drain current ID is:
- 1 mA
- 2 mA (Correct answer)
- 4 mA
- 0.5 mA
Correct answer: 2 mA
Since VDS > VGS − VTN = 2 V, the device is in saturation: ID = (kn/2)(VGS − VTN)² = 0.25 × 4 = 1 mA. Wait — kn already includes the W/L factor, so ID = (kn/2)(VGS−VTN)² = (0.5/2)(2)² = 1 mA.
Question 7: Which of the following correctly describes a Schottky diode compared to a p-n junction diode?
- It has a higher forward voltage drop and slower switching speed
- It has a lower forward voltage drop and faster switching speed (Correct answer)
- It stores more minority carriers during forward bias
- It requires reverse bias to conduct current
Correct answer: It has a lower forward voltage drop and faster switching speed
Schottky diodes use a metal-semiconductor junction with no minority carrier storage, giving lower VF (~0.3 V) and much faster switching.
BEE Semiconductor Devices and Circuits 2
In a MOSFET, the threshold voltage VT is most directly affected by which parameter?