ISCET Semiconductor Devices 3 — Questions and Answers
Question 1: In a MOSFET, the gate terminal is insulated from the channel by which material?
- A thin P-type silicon layer
- Silicon dioxide (SiO2) (Correct answer)
- Germanium film
- A reverse-biased PN junction
Correct answer: Silicon dioxide (SiO2)
The MOSFET gate is separated from the channel by a thin silicon dioxide (SiO2) insulating layer, which is why virtually no gate current flows.
Question 2: A BJT is said to be in saturation when:
- Both the base-emitter and base-collector junctions are forward biased (Correct answer)
- The collector-emitter voltage is at its maximum
- Only the base-emitter junction is forward biased
- The base current is zero
Correct answer: Both the base-emitter and base-collector junctions are forward biased
Saturation occurs when both junctions are forward biased, causing VCE to drop to a low value (typically 0.2 V for silicon) and the transistor acts like a closed switch.
Question 3: What is the primary advantage of a Darlington pair transistor configuration?
- Very high frequency response
- Very high current gain (β² ≈ β1 × β2) (Correct answer)
- Lower noise figure
- Reduced power consumption
Correct answer: Very high current gain (β² ≈ β1 × β2)
A Darlington pair cascades two BJTs so their betas multiply, yielding an extremely high overall current gain equal to approximately β1 × β2.
Question 4: Which type of diode is specifically designed to emit light when forward biased?
- Schottky diode
- Varactor diode
- Light-emitting diode (LED) (Correct answer)
- PIN diode
Correct answer: Light-emitting diode (LED)
LEDs are made from direct-bandgap semiconductors (like GaAs or GaN) where electron-hole recombination releases energy as photons rather than heat.
Question 5: The Early effect in a BJT refers to:
- Increased leakage current at high frequencies
- Narrowing of the base width as VCE increases, causing IC to rise slightly (Correct answer)
- Thermal runaway due to excessive collector current
- Punch-through breakdown at the emitter
Correct answer: Narrowing of the base width as VCE increases, causing IC to rise slightly
The Early effect (base-width modulation) occurs when higher VCE widens the collector depletion region, effectively narrowing the base and slightly increasing IC.
Question 6: In an enhancement-mode N-channel MOSFET, the device is normally:
- On, with a conducting channel at VGS = 0
- Off, and requires a positive VGS above the threshold voltage to create a channel (Correct answer)
- Off, and requires a negative VGS to turn on
- On at all gate voltages above zero
Correct answer: Off, and requires a positive VGS above the threshold voltage to create a channel
Enhancement-mode MOSFETs have no conducting channel at VGS = 0 and require VGS to exceed the positive threshold voltage (VT) to induce a channel.
Question 7: A varactor (varicap) diode is used primarily as a:
- Voltage-controlled capacitor (Correct answer)
- Current-controlled resistor
- Light sensor
- High-speed switch
Correct answer: Voltage-controlled capacitor
A varactor diode exploits the voltage-dependent capacitance of its reverse-biased depletion region, making it useful as a voltage-controlled capacitor in tuning circuits.
ISCET Semiconductor Devices 3
In a MOSFET, the gate terminal is insulated from the channel by which material?