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 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.22Medium
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.23Medium
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.24Medium
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.25Medium
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.26Medium
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.
Q.27Medium
A solar cell's efficiency is reduced by all the following EXCEPT:
Answer: C
Increasing minority carrier lifetime improves efficiency by reducing recombination losses. Other options reduce efficiency.
Q.28Medium
The output impedance of a common-emitter amplifier is determined primarily by:
Answer: A
Output impedance Zo ≈ Rc || RL (Rc in parallel with load resistance), depending on collector circuit configuration.
Q.29Medium
In a MOSFET, the threshold voltage VT is defined as the gate voltage at which:
Answer: B
Threshold voltage VT is when the surface band bending creates an inversion layer, transition from depletion to strong inversion.
Q.30Medium
In a p-n junction under reverse bias, the depletion width increases because:
Answer: A
Reverse bias voltage adds to the built-in potential, increasing the total electric field and widening the depletion region further.
Q.31Medium
What is the primary cause of temperature dependence of bandgap energy in semiconductors?
Answer: B
Temperature affects bandgap through thermal expansion (lattice constant changes) and electron-phonon coupling. The Varshni equation describes this relationship: Eg(T) = Eg(0) - αT²/(T+β).
Q.32Medium
When a p-n junction is forward biased, the width of the depletion region:
Answer: B
Forward bias reduces the effective potential barrier (V₀ - V_f), causing the depletion width W to decrease according to W = √(2εε₀(V₀-V_f)/(qNₐNd/(Nₐ+Nd))).
Q.33Medium
The reverse saturation current (I₀) of a diode doubles approximately every:
Answer: A
I₀ approximately doubles for every 5°C rise in temperature due to exponential increase in intrinsic carrier concentration with temperature. This is critical for thermal management in circuits.
Q.34Medium
What is the typical doping concentration for a lightly doped side of a p-n junction used in photodiodes?
Answer: B
Photodiodes use lightly doped (intrinsic or semi-intrinsic) regions with doping ~10^15 cm^-3 to extend the depletion region and improve light collection efficiency.
Q.35Medium
When a BJT enters saturation, the relationship between collector and base current is best described as:
Answer: B
In saturation, the BJT acts as a closed switch. The collector current is limited by the external circuit (Vcc and Rc) rather than by the base current, making Ic ≤ (Vcc - Vce,sat)/Rc.
Q.36Medium
In a JFET, pinch-off occurs when:
Answer: B
Pinch-off in a JFET occurs when the depletion regions from the p-type gates expand sufficiently to meet at the channel center, effectively closing the conduction path despite further voltage increase.
Q.37Medium
The body effect in a MOSFET (substrate bias effect) occurs because:
Answer: B
Body effect: when substrate is reverse biased, the depletion region widens, increasing the voltage needed to invert the surface. Vt increases by η × Vsb, where η is body effect coefficient.
Q.38Medium
In a metal-semiconductor (Schottky) junction, the barrier height is primarily determined by:
Answer: A
Schottky barrier height Φ_B ≈ Φ_M - χ, where Φ_M is metal work function and χ is semiconductor electron affinity. This makes Schottky junctions useful for various applications with tunable barriers.
Q.39Medium
Compared to a p-n junction diode, a Schottky diode has:
Answer: B
Schottky diodes have lower barrier height (typically 0.3-0.5V vs 0.7V for Si), resulting in lower forward voltage. Faster switching due to majority carrier conduction (no minority carrier storage).
Q.40Medium
In a p-i-n photodiode, the intrinsic region serves to:
Answer: B
The intrinsic (lightly doped) region in a p-i-n diode extends the depletion width, increasing quantum efficiency for photon absorption and collection. This improves responsivity and frequency response.
