CET Semiconductor Devices & Circuits 3 — Questions and Answers
Question 1: What is the depletion region in a p-n junction?
- The region where majority carriers accumulate
- The region devoid of mobile charge carriers near the junction (Correct answer)
- The p-type semiconductor bulk material
- The metal contact layer
Correct answer: The region devoid of mobile charge carriers near the junction
The depletion region forms at the p-n junction when electrons and holes recombine, leaving an area with no free charge carriers.
Question 2: Which of the following describes the operation of an enhancement-mode NMOS transistor when VGS = 0?
- Channel is fully open (ON)
- Channel is partially open
- Channel does not exist (OFF) (Correct answer)
- Device operates in saturation
Correct answer: Channel does not exist (OFF)
An enhancement-mode NMOS has no channel when VGS = 0; a positive gate voltage must exceed the threshold to create the channel.
Question 3: The breakdown mechanism in heavily doped, thin-junction diodes at low reverse voltages is called:
- Avalanche breakdown
- Zener breakdown (Correct answer)
- Thermal runaway
- Punch-through
Correct answer: Zener breakdown
Zener (or tunneling) breakdown occurs in heavily doped junctions where the strong electric field allows electrons to tunnel through the narrow barrier.
Question 4: In a common-emitter amplifier, what is the phase relationship between the input and output signals?
- In-phase (0°)
- 90° leading
- 180° out-of-phase (Correct answer)
- 90° lagging
Correct answer: 180° out-of-phase
The common-emitter configuration inverts the signal, producing a 180° phase shift between base input and collector output.
Question 5: A varactor diode is used primarily as a:
- Variable resistor
- Voltage-controlled capacitor (Correct answer)
- Current-controlled inductor
- Fixed voltage reference
Correct answer: Voltage-controlled capacitor
A varactor (varicap) diode's depletion-region width changes with reverse bias, varying its junction capacitance for tuning applications.
Question 6: What is the pinch-off voltage in a JFET?
- The gate voltage at which the drain current is maximum
- The gate-source voltage that reduces drain current to nearly zero (Correct answer)
- The drain voltage where the channel becomes ohmic
- The supply voltage required to turn on the device
Correct answer: The gate-source voltage that reduces drain current to nearly zero
Pinch-off voltage (VP) is the gate-source voltage at which the depletion regions close the channel, reducing ID to approximately zero.
Question 7: An SCR (Silicon Controlled Rectifier) can be triggered into conduction by:
- Reducing anode voltage below threshold
- Applying a positive pulse to the gate (Correct answer)
- Reversing the anode-cathode polarity
- Removing the gate signal permanently
Correct answer: Applying a positive pulse to the gate
An SCR latches ON when a positive gate trigger pulse fires it while the anode is positive with respect to the cathode.
CET Semiconductor Devices & Circuits 3
What is the depletion region in a p-n junction?