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.
In an avalanche photodiode (APD), the internal gain (multiplication factor M) increases significantly due to:
Answer: B
APD operates in high reverse bias where impact ionization (collision ionization) produces secondary electron-hole pairs, creating avalanche multiplication and signal amplification internally.
Q.122Hard
The specific contact resistance of a metal-semiconductor junction is proportional to:
Answer: B
Contact resistance ρc ∝ exp(φB/kT)/Nc where φB is barrier height. Higher barrier leads to exponentially higher resistance following Thermionic emission theory.
Q.123Hard
Which combination of semiconductor parameters would result in the highest intrinsic carrier concentration at 300K?
Answer: B
ni = √(Nc·Nv)·exp(-Eg/2kT) where Nc, Nv depend on effective masses. Small Eg and high density of states both exponentially increase ni. GaAs has higher ni than Si due to smaller Eg.
Q.124Hard
In a tunnel diode, negative differential resistance occurs because:
Answer: C
In tunnel diode's NDR region, as voltage increases, tunneling current decreases (fewer states to tunnel into) while diffusion current increases, causing net decrease in total current.
Q.125Hard
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.126Hard
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.127Hard
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.128Hard
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.129Hard
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.130Hard
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.131Hard
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.132Hard
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.133Hard
The quantum well structure in modern semiconductors is primarily used for:
Answer: B
Quantum confinement in thin layers (nanometers) modifies the density of states from 3D to 2D structure, changing effective masses and enabling bandgap engineering for applications like quantum well lasers and LEDs.
Q.134Hard
Heterojunctions (like AlGaAs/GaAs) provide advantages over homojunctions primarily because:
Answer: B
Wide bandgap material (AlGaAs) acts as barrier, confining carriers to narrow bandgap GaAs region. This reduces recombination, improves injection efficiency, and is crucial for LEDs and laser diodes.
Q.135Hard
In a Zener diode, breakdown voltage is primarily determined by:
Answer: B
Higher doping concentrations create stronger electric fields at lower voltages, reducing Zener breakdown voltage. This allows designing Zeners with specific breakdown voltages (3.3V, 5.1V, etc.).
Q.136Hard
In a tunnel diode, the negative resistance region occurs due to:
Answer: B
At low forward bias, electrons tunnel through the narrow bandgap, creating peak current. As voltage increases, direct band-to-band current dominates, causing current to decrease with voltage (negative differential resistance).
Q.137Hard
The transconductance parameter μ_n × C_ox in a MOSFET depends on:
Answer: C
The process transconductance parameter k_n = μ_n × C_ox determines how efficiently the MOSFET converts gate voltage to drain current. Both μ_n and C_ox are independent device parameters.
Q.138Hard
In FinFET technology (used in modern 7nm and 5nm nodes), multiple gates are used to:
Answer: B
FinFETs employ multiple gates (typically 3 gates) wrapping around a thin silicon fin, providing superior electrostatic control of the channel, reducing short-channel effects and subthreshold swing.
Q.139Hard
In a CMOS inverter circuit, the propagation delay is minimized when:
Answer: B
Due to lower hole mobility in PMOS (~2-3 times lower than electron mobility in NMOS), the PMOS width must be 2-3 times larger to match switching speeds and minimize inverter delay.
Q.140Hard
In a BJT transistor, the Early effect causes the collector current to increase slightly with increasing reverse bias on the collector-base junction. This is due to:
Answer: B
The Early effect occurs because increased reverse bias widens the depletion region, reducing the effective base width. This decreases recombination in the base and increases collector current. The Early voltage V_A is inversely proportional to base doping concentration.
