When a semiconductor is exposed to light with photon energy greater than band gap energy, the phenomenon is:
Answer: B
Photoconductivity is the increase in conductivity when photons with E > Eg create electron-hole pairs, increasing charge carrier concentration.
Q.722Hard
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.723Medium
In a MOSFET, the threshold voltage VT is defined as the gate voltage at which:
Answer: B
Threshold voltage VT is when the surface band bending creates an inversion layer, transition from depletion to strong inversion.
Q.724Hard
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.725Medium
In a p-n junction under reverse bias, the depletion width increases because:
Answer: A
Reverse bias voltage adds to the built-in potential, increasing the total electric field and widening the depletion region further.
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Q.726Hard
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.727Easy
At room temperature (300 K), the intrinsic carrier concentration of silicon is approximately:
Answer: A
The intrinsic carrier concentration (ni) of silicon at 300 K is approximately 1.5 × 10^10 cm^-3, which is a standard value used in semiconductor calculations.
Q.728Medium
What is the primary cause of temperature dependence of bandgap energy in semiconductors?
Answer: B
Temperature affects bandgap through thermal expansion (lattice constant changes) and electron-phonon coupling. The Varshni equation describes this relationship: Eg(T) = Eg(0) - αT²/(T+β).
Q.729Easy
In a p-n junction at equilibrium, the potential difference across the junction is called:
Answer: B
The built-in potential (V₀) develops naturally across a p-n junction due to diffusion of charge carriers. For silicon at 300 K, V₀ ≈ 0.7 V, independent of external voltage.
Q.730Medium
When a p-n junction is forward biased, the width of the depletion region:
Answer: B
Forward bias reduces the effective potential barrier (V₀ - V_f), causing the depletion width W to decrease according to W = √(2εε₀(V₀-V_f)/(qNₐNd/(Nₐ+Nd))).
Q.731Medium
The reverse saturation current (I₀) of a diode doubles approximately every:
Answer: A
I₀ approximately doubles for every 5°C rise in temperature due to exponential increase in intrinsic carrier concentration with temperature. This is critical for thermal management in circuits.
Q.732Medium
What is the typical doping concentration for a lightly doped side of a p-n junction used in photodiodes?
Answer: B
Photodiodes use lightly doped (intrinsic or semi-intrinsic) regions with doping ~10^15 cm^-3 to extend the depletion region and improve light collection efficiency.
Q.733Hard
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.734Hard
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.735Medium
When a BJT enters saturation, the relationship between collector and base current is best described as:
Answer: B
In saturation, the BJT acts as a closed switch. The collector current is limited by the external circuit (Vcc and Rc) rather than by the base current, making Ic ≤ (Vcc - Vce,sat)/Rc.
Q.736Hard
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.737Medium
In a JFET, pinch-off occurs when:
Answer: B
Pinch-off in a JFET occurs when the depletion regions from the p-type gates expand sufficiently to meet at the channel center, effectively closing the conduction path despite further voltage increase.
Q.738Hard
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.739Hard
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.740Medium
The body effect in a MOSFET (substrate bias effect) occurs because:
Answer: B
Body effect: when substrate is reverse biased, the depletion region widens, increasing the voltage needed to invert the surface. Vt increases by η × Vsb, where η is body effect coefficient.
