ISCET Semiconductor Devices 5 — Questions and Answers
Question 1: A Schottky diode differs from a standard PN junction diode primarily because it:
- Uses two P-type layers
- Forms a metal-semiconductor junction with no minority carrier storage, allowing faster switching (Correct answer)
- Has a higher forward voltage drop of about 1.2 V
- Requires a magnetic field to operate
Correct answer: Forms a metal-semiconductor junction with no minority carrier storage, allowing faster switching
Schottky diodes use a metal-semiconductor junction instead of a PN junction, eliminating minority carrier storage and enabling much faster switching speeds.
Question 2: What does the term 'transconductance' (gm) describe in a FET?
- The ratio of drain voltage to gate voltage
- The ratio of the change in drain current to the change in gate-source voltage (Correct answer)
- The channel resistance at pinch-off
- The gate-to-drain feedback capacitance
Correct answer: The ratio of the change in drain current to the change in gate-source voltage
Transconductance (gm = ΔID/ΔVGS) measures how effectively the gate voltage controls the drain current, and is the key gain parameter for FET amplifier design.
Question 3: A PIN diode is commonly used in RF applications because:
- It acts as a variable capacitor at all frequencies
- Its intrinsic (I) region provides a voltage-controlled resistance at RF frequencies (Correct answer)
- It generates high-frequency oscillations
- It has no junction capacitance
Correct answer: Its intrinsic (I) region provides a voltage-controlled resistance at RF frequencies
The wide intrinsic layer of a PIN diode acts as a variable RF resistance controlled by DC bias current, making it useful for RF switching and attenuation.
Question 4: When a BJT is in the active (linear) region, which junction biasing condition applies?
- Both junctions reverse biased
- Base-emitter forward biased; base-collector reverse biased (Correct answer)
- Both junctions forward biased
- Base-emitter reverse biased; base-collector forward biased
Correct answer: Base-emitter forward biased; base-collector reverse biased
Normal active-region operation requires the base-emitter junction forward biased (VBE ≈ 0.7 V for silicon) and the base-collector junction reverse biased.
Question 5: The maximum reverse voltage a diode can withstand without breakdown is called the:
- Knee voltage
- Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV) (Correct answer)
- Holding voltage
- Forward voltage rating
Correct answer: Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV)
The PIV (or PRV) rating specifies the maximum reverse voltage a diode can block; exceeding it causes destructive avalanche or Zener breakdown.
Question 6: In a JFET, operating the device at VGS = 0 V produces the drain current known as:
- Leakage current IGSS
- Maximum drain current IDSS (Correct answer)
- Pinch-off current IP
- Saturation current IS
Correct answer: Maximum drain current IDSS
IDSS (Drain-Source current with gate Shorted) is the maximum drain current that flows when VGS = 0 V and VDS is in the saturation region.
Question 7: Avalanche breakdown in a reverse-biased diode occurs due to:
- Quantum mechanical tunneling of carriers through the barrier
- High-energy carriers colliding with lattice atoms and creating additional electron-hole pairs (impact ionization) (Correct answer)
- Thermal generation of carriers exceeding recombination rate
- Forward bias exceeding the knee voltage
Correct answer: High-energy carriers colliding with lattice atoms and creating additional electron-hole pairs (impact ionization)
Avalanche breakdown results from impact ionization, where accelerated carriers gain enough energy to knock additional electron-hole pairs free, creating a self-sustaining current multiplication.
ISCET Semiconductor Devices 5
A Schottky diode differs from a standard PN junction diode primarily because it: