Physics is where JEE ranks are usually won or lost, since a single conceptual slip changes the whole answer. Questions here span mechanics, rotational motion, thermodynamics, waves and oscillations, electrostatics, current electricity, magnetism, optics, and modern physics. Numerical problems carry full derivations so you can see which step you skipped, not just which option was right.
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.322Medium
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.323Medium
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.324Medium
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.325Medium
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.326Medium
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.327Medium
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.328Medium
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.329Medium
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.330Medium
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.331Medium
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.332Medium
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.333Medium
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.334Medium
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.
Q.335Medium
In the Einstein relation for semiconductors, the ratio of diffusion coefficient to mobility (D/μ) equals:
Answer: B
Einstein relation: D/μ = kT/e. At room temperature (300K), this thermal voltage ≈ 26 mV. This fundamental relation connects drift and diffusion processes in semiconductors.
Q.336Medium
The temperature coefficient of resistance for semiconductors is:
Answer: B
Semiconductors have negative temperature coefficient because increased temperature increases intrinsic carrier concentration, decreasing resistance.
Q.337Medium
In a metal-semiconductor contact, if the metal work function is greater than the semiconductor work function, a __________ barrier is formed:
Answer: A
When metal work function > semiconductor work function, electrons transfer from semiconductor to metal, creating a Schottky barrier.
Q.338Medium
In an LED, the wavelength of emitted light depends on:
Answer: B
The energy of emitted photons equals the band gap energy (E = Eg = hc/λ), determining the color/wavelength of LED light.
Q.339Medium
The Hall coefficient for a p-type semiconductor is:
Answer: A
Hall coefficient is positive for p-type (hole conduction) and negative for n-type (electron conduction), based on the sign of majority carriers.
Q.340Medium
The transconductance (gm) of a MOSFET increases with:
Answer: A
Transconductance gm ∝ W/L (width-to-length ratio), so decreasing channel length L increases transconductance.
