ISCET Semiconductor Device Principles 3 โ Questions and Answers
Question 1: What is the diffusion capacitance of a forward-biased p-n junction proportional to?
- Reverse bias voltage
- Forward bias current (Correct answer)
- Junction area only
- Doping concentration only
Correct answer: Forward bias current
Diffusion capacitance is proportional to the DC forward bias current because more current means more stored minority charge.
Question 2: In an enhancement-mode N-channel MOSFET, the device is normally:
- ON with zero gate bias and turned OFF by negative VGS
- OFF with zero gate bias and turned ON by positive VGS above threshold (Correct answer)
- ON at all times regardless of gate voltage
- OFF only when VDS exceeds the breakdown voltage
Correct answer: OFF with zero gate bias and turned ON by positive VGS above threshold
Enhancement-mode MOSFETs have no channel at VGS = 0 and require VGS > VTH to invert the channel and allow drain current.
Question 3: What is the transconductance (gm) of a MOSFET in the saturation region given ID = 4 mA and VGS โ VTH = 2 V?
- 2 mA/V
- 4 mA/V (Correct answer)
- 8 mA/V
- 0.5 mA/V
Correct answer: 4 mA/V
In saturation, gm = 2ยทID / (VGS โ VTH) = (2 ร 4 mA) / 2 V = 4 mA/V.
Question 4: Which type of breakdown in a reverse-biased diode is dominant at voltages below approximately 5 V in heavily doped junctions?
- Avalanche breakdown
- Thermal breakdown
- Zener (tunneling) breakdown (Correct answer)
- Punch-through breakdown
Correct answer: Zener (tunneling) breakdown
Zener (band-to-band tunneling) breakdown dominates in heavily doped junctions at low reverse voltages (below ~5 V) where the depletion region is very narrow.
Question 5: The pinch-off voltage (VP) of a JFET is the value of VGS at which:
- The drain current reaches its maximum
- The channel is completely depleted and drain current drops to zero (Correct answer)
- The gate-to-drain junction becomes forward biased
- The device enters saturation
Correct answer: The channel is completely depleted and drain current drops to zero
Pinch-off voltage is the gate-to-source voltage that fully depletes the channel, cutting off drain current (IDSS approaches zero).
Question 6: A silicon BJT has ฮฒ = 100 and IC = 5 mA. What is the approximate base current IB?
- 0.005 mA
- 0.05 mA (Correct answer)
- 0.5 mA
- 5 mA
Correct answer: 0.05 mA
IB = IC / ฮฒ = 5 mA / 100 = 0.05 mA (50 ยตA).
Question 7: What physical phenomenon causes the channel-length modulation effect in short-channel MOSFETs?
- Gate oxide thinning
- Extension of the drain depletion region into the channel, reducing effective channel length (Correct answer)
- Increased doping near the source
- Reduction of carrier mobility at high fields
Correct answer: Extension of the drain depletion region into the channel, reducing effective channel length
As VDS increases beyond pinch-off, the drain depletion region expands toward the source, shortening the effective channel and increasing drain current.
ISCET Semiconductor Device Principles 3
What is the diffusion capacitance of a forward-biased p-n junction proportional to?