ISCET Semiconductor Device Principles 4 ā Questions and Answers
Question 1: What is the purpose of the intrinsic layer in a PIN diode?
- To increase reverse breakdown voltage by concentrating the field
- To provide a wide depletion region that reduces junction capacitance and enables fast switching at RF frequencies (Correct answer)
- To increase forward voltage drop
- To improve thermal conductivity
Correct answer: To provide a wide depletion region that reduces junction capacitance and enables fast switching at RF frequencies
The intrinsic (I) layer widens the depletion region, lowering capacitance and allowing PIN diodes to function as RF switches or attenuators.
Question 2: In a common-emitter BJT amplifier, which parameter is primarily responsible for the upper cutoff frequency limitation at high frequencies?
- Base spreading resistance only
- Miller-multiplied collector-base capacitance (CĀµ) (Correct answer)
- Emitter bypass capacitor
- Power supply impedance
Correct answer: Miller-multiplied collector-base capacitance (CĀµ)
The Miller effect multiplies the collector-base capacitance CĀµ by (1 + |Av|), creating a dominant pole that limits high-frequency response.
Question 3: What does the threshold voltage (VTH) of a MOSFET depend on?
- Drain-to-source voltage and channel length only
- Gate oxide thickness, substrate doping, flat-band voltage, and oxide charge (Correct answer)
- Only the gate oxide thickness
- Channel width and drain current
Correct answer: Gate oxide thickness, substrate doping, flat-band voltage, and oxide charge
VTH is determined by oxide thickness (tox), substrate doping (NA), flat-band voltage (VFB), and interface/oxide charges.
Question 4: A Schottky diode differs from a p-n junction diode primarily because:
- It uses two p-type semiconductor layers
- It is a metal-semiconductor junction with majority carrier conduction and no minority carrier storage (Correct answer)
- It has a higher forward voltage drop than silicon diodes
- It requires a reverse bias to conduct
Correct answer: It is a metal-semiconductor junction with majority carrier conduction and no minority carrier storage
Schottky diodes are metal-semiconductor junctions relying on majority carriers, eliminating minority carrier storage and enabling very fast switching.
Question 5: The small-signal output resistance (ro) of a BJT in the common-emitter configuration is approximately:
- Ī² / gm
- VA / IC (Correct answer)
- 1 / gm
- VT / IC
Correct answer: VA / IC
The output resistance ro ā VA / IC, where VA is the Early voltage and IC is the quiescent collector current.
Question 6: What is the significance of the depletion approximation used in p-n junction analysis?
- It assumes all carriers are mobile throughout the device
- It assumes the depletion region has zero free carriers and the neutral regions have no charge (Correct answer)
- It ignores the effect of doping on junction width
- It assumes temperature has no effect on junction behavior
Correct answer: It assumes the depletion region has zero free carriers and the neutral regions have no charge
The depletion approximation assumes the depletion region is fully depleted of free carriers (abrupt edges) and neutral regions are charge-free, simplifying junction width calculations.
Question 7: In a depletion-mode MOSFET, the device conducts when VGS is:
- Equal to zero only
- Zero or negative (for N-channel), with a pre-existing channel at VGS = 0 (Correct answer)
- Positive only, above threshold
- Only when VDS equals zero
Correct answer: Zero or negative (for N-channel), with a pre-existing channel at VGS = 0
Depletion-mode MOSFETs have a built-in channel that conducts at VGS = 0 and can be pinched off by applying a negative VGS (for N-channel).
ISCET Semiconductor Device Principles 4
What is the purpose of the intrinsic layer in a PIN diode?