In Compton scattering, a photon collides with a free electron at rest. Which quantity always increases?

The correct answer is B: Photon wavelength. In Compton effect, photon transfers energy to electron, losing energy and increasing wavelength. Δλ = (h/m_e c)(1 - cosθ).

At thermal equilibrium in a semiconductor, the product of electron and hole concentrations is:

The correct answer is B: ni² (intrinsic carrier concentration squared). 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.

A ray of light travels from medium 1 (refractive index n₁ = 1.5) to medium 2 (refractive index n₂ = 1.0). If the angle of incidence is 30°, what is the angle of refraction?

The correct answer is A: 48.6°. Using Snell's law: n₁sin(θ₁) = n₂sin(θ₂). So 1.5 × sin(30°) = 1.0 × sin(θ₂). This gives sin(θ₂) = 0.75, therefore θ₂ = 48.6°

The heat capacity at constant pressure Cp is greater than heat capacity at constant volume Cv for ideal gases because:

The correct answer is A: At constant pressure, some energy goes into work expansion. At constant P: Q = ΔU + PΔV, so more heat is needed to produce same temperature rise. Relation: Cp - Cv = R

The pinch-off voltage Vp in a JFET is the gate voltage at which:

The correct answer is A: The channel completely depletes and current becomes zero. At pinch-off voltage, the depletion region extends completely across the channel width, cutting off the flow of carriers and reducing drain current to nearly zero (IDSS becomes zero).

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JEE Physics
Semiconductors

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

27 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 21–27 of 27

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.21 Hard Semiconductors

The fill factor of a solar cell is defined as:

A Ratio of maximum power to product of Voc and Isc

B Ratio of efficiency to bandgap energy

C Ratio of short circuit current to open circuit voltage

D Product of voltage and current at maximum power point

Correct Answer: A. Ratio of maximum power to product of Voc and Isc

EXPLANATION

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.

Test

Q.22 Hard Semiconductors

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)

A 2 times

B 10 times

C 100 times

D 1000 times

Correct Answer: D. 1000 times

EXPLANATION

Forward current I = I₀exp(eV/kT). Ratio = exp(e×0.3/kT) = exp(0.3/0.026) ≈ exp(11.5) ≈ 10^5. For ΔV = 0.3V more, increase is exp(11.5) ≈ 1000 times.

Test

Q.23 Hard Semiconductors

In a heterojunction like AlGaAs/GaAs, the main advantage is:

A Reduced lattice mismatch strain

B Carrier confinement due to larger bandgap material creating potential wells

C Increased doping concentration

D Lower operating temperature requirement

Correct Answer: B. Carrier confinement due to larger bandgap material creating potential wells

EXPLANATION

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.

Test

Q.24 Hard Semiconductors

The temperature coefficient of bandgap for silicon is approximately:

A Positive: +2 meV/K

B Negative: -2 meV/K

C Zero at 300 K

D Depends only on doping concentration

Correct Answer: B. Negative: -2 meV/K

EXPLANATION

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.

Test

Q.25 Hard Semiconductors

A depletion mode MOSFET differs from enhancement mode in that:

A It conducts with zero gate voltage

B It has higher breakdown voltage

C It has positive threshold voltage

D It requires higher supply voltage

Correct Answer: A. It conducts with zero gate voltage

EXPLANATION

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.

Test

Q.26 Hard Semiconductors

Impurity scattering dominates in semiconductors when:

A Temperature is very low and doping concentration is high

B Temperature is very high

C Doping concentration is low

D Only at bandgap temperature

Correct Answer: A. Temperature is very low and doping concentration is high

EXPLANATION

At low temperatures, phonon scattering is suppressed, but impurity scattering (interaction with ionized donors/acceptors) remains, limiting mobility. At high T, phonon scattering dominates.

Test

Q.27 Hard Semiconductors

In a MOSFET, the threshold voltage (Vth) increases when:

A Oxide thickness decreases

B Oxide thickness increases

C Doping concentration of substrate decreases

D Temperature increases significantly

Correct Answer: B. Oxide thickness increases

EXPLANATION

Vth = Vfb + 2φF + (√(2εsqNa(2φF))/Cox). Threshold voltage increases with oxide thickness (Cox decreases) and increases with substrate doping. Temperature has weak effect.

Test

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Semiconductors