ISCET Semiconductor Devices 4 — Questions and Answers
Question 1: What is thermal runaway in a BJT circuit, and what component is commonly used to prevent it?
- Oscillation at high frequency; prevented by a bypass capacitor
- Uncontrolled increase in IC due to temperature rise; prevented by emitter resistor (RE) (Correct answer)
- Reverse breakdown; prevented by a Zener clamp
- Base current cutoff; prevented by voltage divider bias
Correct answer: Uncontrolled increase in IC due to temperature rise; prevented by emitter resistor (RE)
Thermal runaway occurs because rising temperature increases IC, which further heats the device; an emitter resistor provides negative feedback to stabilize the operating point.
Question 2: The SCR (Silicon Controlled Rectifier) will turn OFF when:
- A negative pulse is applied to the gate
- The anode current drops below the holding current (IH) (Correct answer)
- The gate signal is removed
- The anode voltage is doubled
Correct answer: The anode current drops below the holding current (IH)
Once triggered, an SCR latches on and only turns off when the anode current falls below the minimum holding current (IH), regardless of the gate signal.
Question 3: In a common-emitter BJT amplifier, what is the phase relationship between the input and output voltages?
- In phase (0°)
- 90° phase shift
- 180° phase inversion (Correct answer)
- 270° phase shift
Correct answer: 180° phase inversion
The common-emitter configuration inverts the signal — a positive-going input produces a negative-going output — resulting in a 180° phase shift.
Question 4: What is the primary difference between a depletion-mode and an enhancement-mode MOSFET?
- Depletion-mode has a physical channel at VGS = 0; enhancement-mode does not (Correct answer)
- Enhancement-mode can handle higher power
- Depletion-mode requires a larger threshold voltage
- Enhancement-mode uses P-type substrate only
Correct answer: Depletion-mode has a physical channel at VGS = 0; enhancement-mode does not
Depletion-mode MOSFETs have a physically doped channel that exists at VGS = 0, while enhancement-mode MOSFETs require gate voltage to induce a channel.
Question 5: Which semiconductor device is a four-layer PNPN structure that acts as a bidirectional switch?
- SCR
- UJT
- TRIAC (Correct answer)
- DIAC
Correct answer: TRIAC
A TRIAC is a bidirectional thyristor that can conduct in both directions and is triggered by either polarity of gate signal, making it ideal for AC power control.
Question 6: The knee voltage of a diode's I-V characteristic represents:
- The reverse breakdown voltage
- The voltage at which forward current increases rapidly (Correct answer)
- The maximum allowable forward current
- The peak inverse voltage rating
Correct answer: The voltage at which forward current increases rapidly
The knee voltage (approximately 0.7 V for silicon) is the forward bias point where the diode transitions from high to very low resistance and current increases sharply.
Question 7: In a UJT (Unijunction Transistor), conduction begins when the emitter voltage reaches:
- The supply voltage VBB
- The peak-point voltage VP, which equals ηVBB + VD (Correct answer)
- Zero volts
- The valley voltage VV
Correct answer: The peak-point voltage VP, which equals ηVBB + VD
The UJT fires (turns on) when the emitter voltage reaches VP = ηVBB + VD, where η is the intrinsic standoff ratio and VD is the diode drop (~0.7 V).
ISCET Semiconductor Devices 4
What is thermal runaway in a BJT circuit, and what component is commonly used to prevent it?