GROL Semiconductor and Tube Components 3 — Questions and Answers
Question 1: For normal operation of a JFET, the gate-source junction must be:
- Reverse biased to control the depletion region width and channel conductivity (Correct answer)
- Forward biased to increase channel conductivity
- At exactly zero bias with no DC component allowed
- Alternately forward and reverse biased by the signal only
Correct answer: Reverse biased to control the depletion region width and channel conductivity
A JFET uses reverse bias on the gate-source P-N junction to widen the depletion region into the channel, narrowing the conductive channel and reducing drain current without drawing significant gate current.
Question 2: An N-channel enhancement-mode MOSFET (E-MOSFET) differs from a depletion-mode MOSFET in that it:
- Contains P-type carriers as the majority carriers in the channel
- Has no pre-formed channel and requires a positive VGS above threshold voltage to induce a conducting inversion layer (Correct answer)
- Can only operate in the ohmic (triode) region
- Has a lower threshold voltage than depletion-mode devices
Correct answer: Has no pre-formed channel and requires a positive VGS above threshold voltage to induce a conducting inversion layer
Enhancement-mode MOSFETs are 'normally off' devices with no implanted channel; a gate voltage above threshold attracts electrons to form an inversion layer between drain and source.
Question 3: Which diode type is used as a voltage-variable capacitor in voltage-controlled oscillators (VCOs) and electronically tunable filters?
- Zener diode
- Schottky barrier diode
- Varactor (varicap) diode (Correct answer)
- Tunnel diode
Correct answer: Varactor (varicap) diode
A reverse-biased varactor diode's junction capacitance varies predictably with the applied reverse voltage, enabling electronic tuning of resonant circuits without mechanical adjustments.
Question 4: A tunnel diode exhibits a negative-resistance region in its I-V characteristic because:
- Lightly doped regions allow tunneling at low reverse voltages producing excess current
- Heavy P+ and N+ doping creates a very thin junction through which electrons tunnel quantum-mechanically at low forward voltages; current decreases past the peak as tunneling probability falls (Correct answer)
- The junction operates continuously in avalanche breakdown producing oscillations
- The wide depletion region stores excess charge that discharges in the negative-resistance region
Correct answer: Heavy P+ and N+ doping creates a very thin junction through which electrons tunnel quantum-mechanically at low forward voltages; current decreases past the peak as tunneling probability falls
Degenerate (very heavy) doping thins the depletion region enough for quantum tunneling; as forward bias increases past the peak, energy alignment for tunneling is lost, causing current to drop — negative resistance.
Question 5: The wide intrinsic (I) layer in a PIN diode is primarily included to:
- Increase reverse breakdown voltage for high-power DC rectification
- Raise maximum operating temperature by reducing thermal resistance
- Minimize junction capacitance at RF and microwave frequencies, enabling the device to function as a voltage-controlled switch or attenuator (Correct answer)
- Provide a large photon-emitting area for LED applications
Correct answer: Minimize junction capacitance at RF and microwave frequencies, enabling the device to function as a voltage-controlled switch or attenuator
The I layer widens the depletion region to minimize capacitance at RF frequencies; DC forward bias controls the conductivity of the I layer, making the PIN diode an effective microwave switch.
Question 6: The lower forward voltage drop (~0.3 V) of a Schottky diode compared to a silicon P-N diode (~0.7 V) is due to:
- Lower doping concentration in the anode region reducing the built-in potential
- The thermionic emission mechanism at the metal-semiconductor interface, which has a lower barrier potential than a P-N homojunction (Correct answer)
- Minority carrier storage in the intrinsic layer that offsets recombination losses
- The use of germanium rather than silicon for the semiconductor material
Correct answer: The thermionic emission mechanism at the metal-semiconductor interface, which has a lower barrier potential than a P-N homojunction
Current in a Schottky diode flows by majority-carrier thermionic emission over a metal-semiconductor barrier that is lower than a silicon P-N junction, and there is no minority carrier storage, yielding very fast switching.
Question 7: In a JFET datasheet, IDSS is defined as:
- Maximum allowable drain-to-source voltage at cutoff
- Gate-to-source reverse leakage current at rated voltage
- Minimum drain current required to maintain the channel in saturation
- Drain current measured with gate shorted to source (VGS = 0), representing the maximum drain current for a depletion-mode device (Correct answer)
Correct answer: Drain current measured with gate shorted to source (VGS = 0), representing the maximum drain current for a depletion-mode device
IDSS (Drain current, Source shorted to gate) specifies the drain current when VGS = 0 V and is used as the reference maximum current from which transconductance curves are plotted.
GROL Semiconductor and Tube Components 3
For normal operation of a JFET, the gate-source junction must be: