In n-type semiconductors, donor levels introduce electrons near the conduction band, shifting the Fermi level upward, making it closer to the conduction band than to the valence band.
Q.2Medium
The conductivity of a semiconductor increases with temperature because:
Answer: D
While mobility decreases with temperature due to increased phonon scattering, the exponential increase in intrinsic carrier concentration dominates, resulting in net increase in conductivity. However, (b) is the primary reason.
Q.3Medium
The intrinsic carrier concentration ni in a semiconductor is given by: ni = √(NcNv)exp(-Eg/2kT). What does Nc represent?
Answer: B
Nc is the effective density of states in the conduction band, which depends on the effective mass of electrons and temperature. Similarly, Nv is for the valence band.
Q.4Medium
In forward biasing of a p-n junction, the depletion region width:
Answer: B
Forward bias reduces the potential barrier at the junction, allowing carriers to move across more easily. This reduces the depletion region width, which is inversely related to the applied voltage.
Q.5Medium
The reverse saturation current in a p-n junction diode depends on:
Answer: B
Reverse saturation current I0 is determined by intrinsic carrier concentration (ni), diffusion coefficient (D), and junction properties. It is independent of reverse voltage but depends strongly on temperature.
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Q.6Medium
Zener breakdown in a semiconductor occurs when:
Answer: B
Zener breakdown involves quantum mechanical tunneling of electrons directly from valence band to conduction band under strong reverse electric field. It occurs in heavily doped junctions at lower voltages than avalanche breakdown.
Q.7Medium
At thermal equilibrium in a semiconductor, the product of electron and hole concentrations is:
Answer: B
The mass action law states ne·nh = ni² at thermal equilibrium, regardless of doping type. This is a fundamental relationship derived from Fermi-Dirac statistics.
Q.8Medium
A BJT transistor operates in saturation region when:
Answer: B
In saturation, both junctions are forward biased, allowing maximum current flow. In active region, BE is forward and CB is reverse biased. In cutoff, both are reverse biased.
Q.9Medium
The Hall effect coefficient (RH) for an n-type semiconductor is:
Answer: B
For n-type semiconductors, charge carriers are electrons (negative), so the Hall coefficient is negative. For p-type (holes), it is positive. The sign determines carrier type.
Q.10Medium
The photo-generated current in a solar cell is proportional to:
Answer: B
Photo-generated current IL is directly proportional to incident light intensity (photon flux). At open circuit, the current is independent of applied voltage; voltage dependence matters for load characteristics.
Q.11Medium
The conductivity of a semiconductor is given by σ = e(neμe + nhμh). If temperature increases from 300K to 400K, which factor primarily determines the change in conductivity?
Answer: A
Though mobility decreases with temperature (T^-23), the exponential increase in carrier concentration (proportional to exp(-Eg/2kT)) dominates, resulting in net increase in conductivity.
Q.12Medium
In a reverse-biased p-n junction, the depletion width increases when:
Answer: B
Increasing reverse bias voltage creates a stronger electric field, pushing charge carriers away from the junction and widening the depletion region according to W ∝ √V.
Q.13Medium
A silicon semiconductor has ni = 1.5 × 10^10 cm^-3 at 300K. If doped with 10^16 cm^-3 donor atoms, calculate the hole concentration at thermal equilibrium.
Answer: B
Using mass action law: ne·nh = ni². Since ne ≈ Nd = 10^16 cm^-3, then nh = ni²/ne = (1.5×10^10)²/10^16 = 2.25×10^4 cm^-3
Q.14Medium
The reverse saturation current (I₀) in a p-n junction increases exponentially with temperature. This is primarily because:
Answer: A
I₀ ∝ ni² which increases exponentially with temperature. This thermal generation of minority carriers in the neutral regions near the junction constitutes the reverse saturation current.
Q.15Medium
In a Zener diode, the Zener breakdown occurs due to:
Answer: B
Zener breakdown (sharp, reversible breakdown at lower voltages) occurs when a strong electric field in the depletion region enables direct tunneling of electrons across the narrow bandgap.
Q.16Medium
A compound semiconductor like GaAs has a direct bandgap while Si has an indirect bandgap. Which statement about light emission is correct?
Answer: B
Direct bandgap semiconductors like GaAs allow radiative recombination (photon emission) without phonon assistance, making them ideal for LEDs and lasers.
Q.17Medium
The minority carrier diffusion length Ln in a semiconductor depends on which parameters?
Answer: B
Diffusion length Ln = √(Dn·τn) where Dn is diffusion coefficient and τn is minority carrier lifetime. It represents the average distance a carrier travels before recombination.
Q.18Medium
In an n-channel JFET (Junction Field Effect Transistor), when the gate-source voltage (Vgs) becomes more negative, what happens to the channel conductance?
Answer: B
More negative Vgs increases the reverse bias on the gate junction, expanding the depletion region and narrowing the conducting channel, thus decreasing conductance.
Q.19Medium
A photodiode operates in reverse bias to:
Answer: B
Reverse bias widens the depletion region, allowing photogenerated carriers throughout this region to be swept out by the electric field, improving collection efficiency and response time.
Q.20Medium
In a solar cell, the maximum power output occurs when the operating point satisfies which condition?
Answer: C
Maximum power point (MPP) occurs at the knee of the I-V characteristic where the product V×I is maximum, typically at ~80% of Voc and ~90% of Isc.
