BEE Semiconductor Devices and Electronics 5 — Questions and Answers
Question 1: Which semiconductor device is best suited for high-power, high-frequency RF amplification due to its superior electron mobility?
- Silicon BJT
- GaAs MESFET (Correct answer)
- CMOS inverter
- Silicon SCR
Correct answer: GaAs MESFET
GaAs MESFETs leverage GaAs's high electron mobility (~8500 cm²/V·s vs. Si's ~1400) for superior high-frequency performance.
Question 2: The gain-bandwidth product (ft) of a BJT is determined by:
- The DC current gain β alone
- The emitter-collector capacitance only
- The transit frequency where current gain falls to unity (Correct answer)
- The collector supply voltage
Correct answer: The transit frequency where current gain falls to unity
ft is the frequency at which the small-signal current gain magnitude falls to 1, characterizing the transistor's high-frequency limit.
Question 3: In a MOSFET, subthreshold conduction refers to:
- Drain current flowing even when VGS < VT (Correct answer)
- Current flow through the gate oxide
- Source-to-drain punch-through
- Channel pinch-off at VDS = VGS − VT
Correct answer: Drain current flowing even when VGS < VT
Subthreshold current flows via diffusion even below VT and is characterized by the subthreshold slope (~60 mV/decade at room temperature).
Question 4: The depletion approximation in p-n junction analysis assumes:
- Carrier concentrations vary linearly across the junction
- The depletion region has abrupt boundaries with zero free carriers inside (Correct answer)
- Minority carriers dominate throughout the semiconductor
- The electric field is uniform everywhere
Correct answer: The depletion region has abrupt boundaries with zero free carriers inside
The depletion approximation treats the depletion region as having abrupt edges with all mobile carriers absent, simplifying Poisson's equation.
Question 5: A thyristor (SCR) can be turned OFF by:
- Applying a negative gate pulse
- Reducing anode current below the holding current (Correct answer)
- Increasing the gate voltage
- Reverse biasing the gate terminal
Correct answer: Reducing anode current below the holding current
Once latched, an SCR can only be turned off by reducing anode current below the holding current IH, not by gate control.
Question 6: In an n-type semiconductor, adding acceptor impurities causes the Fermi level to:
- Move closer to the conduction band
- Move closer to the valence band (Correct answer)
- Remain at the intrinsic level
- Move above the conduction band
Correct answer: Move closer to the valence band
Acceptors compensate donor electrons, reducing n-type character and shifting the Fermi level toward the valence band.
Question 7: The Miller effect in an amplifier circuit causes the effective input capacitance to be:
- Reduced by the voltage gain
- Multiplied by (1 + |Av|) (Correct answer)
- Equal to the output capacitance
- Divided by the transistor beta
Correct answer: Multiplied by (1 + |Av|)
Miller's theorem shows that a feedback capacitance Cf appears as (1 + |Av|)·Cf at the input, greatly increasing effective input capacitance.
BEE Semiconductor Devices and Electronics 5
Which semiconductor device is best suited for high-power, high-frequency RF amplification due to its superior electron mobility?