BEE BEE Bachelor of Electrical Engineering Semiconductor Devices and Electronics Questions and Answers 3 — Questions and Answers
Question 1: What determines the width of the depletion region in a reverse-biased p-n junction?
- Applied reverse voltage and doping concentrations (Correct answer)
- Temperature only
- Forward current magnitude
- Minority carrier lifetime
Correct answer: Applied reverse voltage and doping concentrations
The depletion width increases with higher reverse voltage and decreases with higher doping concentrations on either side of the junction.
Question 2: In a common-emitter BJT amplifier, what is the phase relationship between the input and output signals?
- 180 degrees out of phase (Correct answer)
- In phase
- 90 degrees out of phase
- 45 degrees out of phase
Correct answer: 180 degrees out of phase
A common-emitter amplifier inverts the signal, producing an output that is 180 degrees out of phase with the input.
Question 3: What is the dominant carrier transport mechanism in the base region of a BJT operating in the active mode?
- Diffusion (Correct answer)
- Drift
- Tunneling
- Thermionic emission
Correct answer: Diffusion
In the narrow base region of a BJT, minority carriers injected from the emitter travel primarily by diffusion due to the concentration gradient.
Question 4: Which parameter of a MOSFET is defined as the ratio of change in drain current to change in gate-source voltage?
- Transconductance (gm) (Correct answer)
- Output conductance (gds)
- Threshold voltage (Vth)
- Channel length modulation factor (λ)
Correct answer: Transconductance (gm)
Transconductance (gm) measures how effectively the gate voltage controls the drain current and is defined as ∂ID/∂VGS.
Question 5: What type of recombination dominates in indirect bandgap semiconductors like silicon?
- Shockley-Read-Hall recombination via trap states (Correct answer)
- Direct band-to-band radiative recombination
- Auger recombination
- Surface recombination only
Correct answer: Shockley-Read-Hall recombination via trap states
In indirect bandgap semiconductors, direct radiative recombination is unlikely, so recombination occurs primarily through trap states in the bandgap via the Shockley-Read-Hall mechanism.
Question 6: What happens to the drain current of an enhancement-mode NMOS transistor when VGS is below the threshold voltage?
- The transistor is in cutoff with essentially zero drain current (Correct answer)
- The transistor operates in the linear region
- The transistor is in saturation with maximum current
- The transistor conducts through the body diode
Correct answer: The transistor is in cutoff with essentially zero drain current
When VGS is below Vth in an enhancement-mode NMOS, no inversion channel forms and the transistor remains in cutoff with negligible drain current.
BEE BEE Bachelor of Electrical Engineering Semiconductor Devices and Electronics Questions and Answers 3
What determines the width of the depletion region in a reverse-biased p-n junction?