BEE Semiconductor Devices and Circuits 4 — Questions and Answers
Question 1: What is the effect of increasing temperature on the reverse saturation current IS of a p-n junction diode?
- IS decreases exponentially
- IS remains constant
- IS increases approximately doubling every 10°C (Correct answer)
- IS increases linearly
Correct answer: IS increases approximately doubling every 10°C
Reverse saturation current approximately doubles for every 10°C rise in temperature due to increased minority carrier generation.
Question 2: In a common-emitter BJT amplifier, adding an unbypassed emitter resistor RE primarily:
- Increases voltage gain and reduces input impedance
- Stabilizes the Q-point and increases input impedance but reduces voltage gain (Correct answer)
- Has no effect on the small-signal voltage gain
- Causes the transistor to enter saturation
Correct answer: Stabilizes the Q-point and increases input impedance but reduces voltage gain
An unbypassed RE provides negative feedback that stabilizes bias, raises input impedance, but reduces the voltage gain from −RC/re to approximately −RC/RE.
Question 3: The output resistance of an ideal op-amp is:
- Infinite
- Zero (Correct answer)
- Equal to the source resistance
- Equal to 50 Ω
Correct answer: Zero
An ideal op-amp has zero output resistance, meaning it can source or sink any current without a voltage drop.
Question 4: Channel length modulation in a MOSFET is modeled in the saturation region by including which factor in the drain current equation?
- (1 + λVDS) (Correct answer)
- (1 − λVGS)
- (1 + λVGS)
- (1 − λVDS)
Correct answer: (1 + λVDS)
The channel-length modulation factor (1 + λVDS) is multiplied into the saturation current to model the slight increase in ID with VDS.
Question 5: Which breakdown mechanism dominates in a heavily doped p-n junction with a thin depletion region at low reverse voltages?
- Avalanche breakdown
- Thermal runaway
- Zener (tunneling) breakdown (Correct answer)
- Punch-through
Correct answer: Zener (tunneling) breakdown
In heavily doped, narrow depletion junctions, quantum mechanical tunneling (Zener effect) dominates at low reverse voltages (typically below 5–6 V).
Question 6: In a half-wave rectifier circuit, the peak inverse voltage (PIV) across the diode is equal to:
- Vpeak/2
- 2 × Vpeak
- Vpeak (Correct answer)
- Vpeak × √2
Correct answer: Vpeak
During the negative half-cycle, the entire peak secondary voltage appears across the reverse-biased diode, so PIV = Vpeak.
Question 7: A silicon BJT has VBE = 0.7 V at IC = 1 mA and T = 300 K. If IC is increased to 10 mA, the new VBE is approximately:
- 0.758 V (Correct answer)
- 0.700 V
- 0.630 V
- 1.400 V
Correct answer: 0.758 V
ΔVBE = VT × ln(10) ≈ 26 mV × 2.303 ≈ 59.8 mV, so new VBE ≈ 0.7 + 0.058 ≈ 0.758 V.
BEE Semiconductor Devices and Circuits 4
What is the effect of increasing temperature on the reverse saturation current IS of a p-n junction diode?