Which of the following is a direct bandgap semiconductor?
Answer: A
GaAs is a direct bandgap semiconductor where the minimum energy gap occurs at the same k-value, making it suitable for light emission. Si and Ge are indirect bandgap semiconductors.
Q.2Easy
At absolute zero temperature, the number of free electrons in an intrinsic semiconductor is:
Answer: B
At T = 0 K, all electrons remain in the valence band. No thermal energy is available to excite electrons to the conduction band, so free electrons are zero.
Q.3Easy
The bandgap of silicon at room temperature (300 K) is approximately:
Answer: B
Silicon has a bandgap of approximately 1.1 eV at 300 K. This is a standard value used in semiconductor physics and device design.
Q.4Medium
In an n-type semiconductor, the Fermi level lies:
Answer: B
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.5Medium
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.
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Q.6Easy
A p-type semiconductor is created by doping silicon with:
Answer: C
Boron is a Group III element (trivalent) that acts as an acceptor in silicon, creating holes and forming p-type semiconductor. Phosphorus, Arsenic, and Antimony are Group V elements (pentavalent) forming n-type semiconductors.
Q.7Medium
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.8Medium
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.9Medium
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.
Q.10Medium
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.11Medium
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.12Medium
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.13Medium
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.14Hard
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.15Easy
An LED emits light because:
Answer: A
In LEDs (forward biased p-n junctions in direct bandgap semiconductors like GaAs), electron-hole recombination releases energy as photons. Indirect bandgap materials (Si, Ge) produce mainly heat.
Q.16Medium
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.17Hard
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.18Hard
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.19Hard
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.20Hard
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.
