BEE Semiconductor Devices and Electronics 4 — Questions and Answers
Question 1: The channel length modulation parameter λ in a MOSFET affects which characteristic?
- The threshold voltage VT
- The slope of ID vs. VDS in the saturation region (Correct answer)
- The oxide capacitance Cox
- The body effect coefficient γ
Correct answer: The slope of ID vs. VDS in the saturation region
Channel length modulation (λ) causes a slight increase in drain current with VDS in saturation, giving ID a finite slope rather than being flat.
Question 2: In a common-base BJT amplifier, the current gain (α) is always:
- Greater than 1
- Equal to the beta (β)
- Less than 1 but close to 1 (Correct answer)
- Negative
Correct answer: Less than 1 but close to 1
Alpha α = IC/IE < 1 because IE = IC + IB and some base current is lost, but α is typically 0.95–0.999.
Question 3: A Schottky diode differs from a standard p-n junction diode primarily because it:
- Uses a metal-semiconductor junction and has faster switching with lower forward voltage (Correct answer)
- Has a higher forward voltage drop (~0.7V)
- Stores significant minority carrier charge
- Is made from compound semiconductors only
Correct answer: Uses a metal-semiconductor junction and has faster switching with lower forward voltage
Schottky diodes form metal-semiconductor junctions, resulting in majority-carrier conduction, ~0.3 V forward drop, and very fast switching.
Question 4: The substrate bias effect (body effect) in a MOSFET causes the threshold voltage to:
- Decrease as substrate reverse bias increases
- Increase as substrate reverse bias increases (Correct answer)
- Remain constant regardless of substrate bias
- Decrease as channel length decreases
Correct answer: Increase as substrate reverse bias increases
Increased reverse substrate bias widens the depletion region, requiring more gate voltage to invert the channel, raising VT.
Question 5: Which configuration of an op-amp has the highest input impedance and unity voltage gain?
- Inverting amplifier
- Differentiator
- Voltage follower (unity-gain buffer) (Correct answer)
- Summing amplifier
Correct answer: Voltage follower (unity-gain buffer)
The voltage follower connects the output directly to the inverting input, achieving unity gain with ideally infinite input impedance.
Question 6: In a semiconductor, the Fermi level in an intrinsic material lies:
- At the conduction band edge
- At the valence band edge
- Near the middle of the bandgap (Correct answer)
- Above the conduction band
Correct answer: Near the middle of the bandgap
In an intrinsic semiconductor, the Fermi level (intrinsic level Ei) lies approximately at the midgap due to equal electron and hole concentrations.
Question 7: The diffusion capacitance of a forward-biased p-n junction is due to:
- Charge stored in the depletion region
- Minority carrier charge stored in the quasi-neutral regions (Correct answer)
- The oxide capacitance at the junction
- Fixed ionized impurity charges
Correct answer: Minority carrier charge stored in the quasi-neutral regions
Diffusion capacitance arises from excess minority carriers injected and stored in the bulk regions adjacent to the junction under forward bias.
BEE Semiconductor Devices and Electronics 4
The channel length modulation parameter λ in a MOSFET affects which characteristic?