In a MOSFET, the threshold voltage (Vth) increases when:
Answer: B
Vth = Vfb + 2φF + (√(2εsqNa(2φF))/Cox). Threshold voltage increases with oxide thickness (Cox decreases) and increases with substrate doping. Temperature has weak effect.
Q.2Hard
Impurity scattering dominates in semiconductors when:
Answer: A
At low temperatures, phonon scattering is suppressed, but impurity scattering (interaction with ionized donors/acceptors) remains, limiting mobility. At high T, phonon scattering dominates.
Q.3Hard
A depletion mode MOSFET differs from enhancement mode in that:
Answer: A
Depletion mode MOSFETs have a conducting channel at Vgs = 0 and require gate voltage to turn OFF. Enhancement mode requires positive gate voltage to create a channel. This is a key structural difference.
Q.4Hard
The temperature coefficient of bandgap for silicon is approximately:
Answer: B
Bandgap of semiconductors decreases with increasing temperature. For Si: dEg/dT ≈ -2.3 meV/K near 300 K. This is derived from Varshni equation and is crucial for device design.
Q.5Hard
In a heterojunction like AlGaAs/GaAs, the main advantage is:
Answer: B
Heterojunctions use materials with different bandgaps to create band offsets that confine carriers (electrons and holes) to specific regions, improving device efficiency. AlGaAs has larger Eg than GaAs, confining carriers in GaAs.
Advertisement
Q.6Hard
In a forward-biased p-n junction diode at room temperature, if the applied voltage is increased from 0.3V to 0.6V, by approximately what factor does the forward current increase? (assume kT/e ≈ 26mV)
Answer: D
Forward current I = I₀exp(eV/kT). Ratio = exp(e×0.3/kT) = exp(0.03.026) ≈ exp(11.5) ≈ 10^5. For ΔV = 0.3V more, increase is exp(11.5) ≈ 1000 times.
Q.7Hard
The fill factor of a solar cell is defined as:
Answer: A
Fill Factor (FF) = Pmax/(Voc × Isc) = (Vm × Im)/(Voc × Isc). It indicates how close the I-V curve is to a rectangle, typically 0.7-0.85 for practical solar cells.
Q.8Hard
In an avalanche photodiode (APD), the internal gain (multiplication factor M) increases significantly due to:
Answer: B
APD operates in high reverse bias where impact ionization (collision ionization) produces secondary electron-hole pairs, creating avalanche multiplication and signal amplification internally.
Q.9Hard
The specific contact resistance of a metal-semiconductor junction is proportional to:
Answer: B
Contact resistance ρc ∝ exp(φB/kT)/Nc where φB is barrier height. Higher barrier leads to exponentially higher resistance following Thermionic emission theory.
Q.10Hard
Which combination of semiconductor parameters would result in the highest intrinsic carrier concentration at 300K?
Answer: B
ni = √(Nc·Nv)·exp(-Eg/2kT) where Nc, Nv depend on effective masses. Small Eg and high density of states both exponentially increase ni. GaAs has higher ni than Si due to smaller Eg.
Q.11Hard
In a tunnel diode, negative differential resistance occurs because:
Answer: C
In tunnel diode's NDR region, as voltage increases, tunneling current decreases (fewer states to tunnel into) while diffusion current increases, causing net decrease in total current.
Q.12Hard
The current gain (β) of a BJT at constant IC depends on:
Answer: B
β varies with temperature and VBE changes, following the Ebers-Moll model. Early voltage causes slight VCE dependence.
Q.13Hard
The noise figure of a semiconductor amplifier is lowest when operating at:
Answer: B
Minimum noise figure occurs at optimum source impedance that matches the device's noise characteristics, typically provided in device specifications.
Q.14Hard
The Shockley ideal diode equation predicts that reverse saturation current (Is) is proportional to:
Answer: C
Is ∝ ni²/(NA·ND), depending on intrinsic carrier concentration squared and inversely on doping concentrations.
Q.15Hard
In a Zener diode, negative resistance occurs in the breakdown region because:
Answer: B
In Zener breakdown, as the reverse bias increases slightly, the breakdown mechanism (avalanche or tunneling) generates more current, but the voltage across the junction decreases due to voltage regulation.
Q.16Hard
The Zener voltage of a heavily doped p-n junction is typically lower than that of a lightly doped junction because:
Answer: B
Heavy doping narrows the depletion region, allowing quantum tunneling at lower voltages. This shifts breakdown mechanism from avalanche (Zener ~5-6V) to tunneling (~3-4V) in heavily doped junctions.
Q.17Hard
The Early voltage (V_A) of a BJT is inversely related to:
Answer: A
Early voltage characterizes the output resistance of a BJT. V_A is inversely proportional to the base width modulation effect (Early effect), which becomes significant in short-base transistors.
Q.18Hard
The subthreshold swing (SS) of a MOSFET is defined as the change in gate voltage required to change drain current by one decade. For an ideal MOSFET, SS at room temperature is approximately:
Answer: B
Ideal subthreshold swing SS = (kT/q) × ln(10) ≈ 60 mV/decade at 300 K. This is a fundamental limit based on the thermal voltage. Real MOSFETs have SS > 60 mV/decade due to interface states.
Q.19Hard
Channel length modulation in a MOSFET leads to:
Answer: B
Channel length modulation occurs when the depletion region at the drain expands with increasing Vds, shortening the effective channel length. This causes output current to increase with voltage, reducing output impedance (ro decreases).
Q.20Hard
The quantum well structure in modern semiconductors is primarily used for:
Answer: B
Quantum confinement in thin layers (nanometers) modifies the density of states from 3D to 2D structure, changing effective masses and enabling bandgap engineering for applications like quantum well lasers and LEDs.
