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.
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.342Medium
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.343Medium
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.344Medium
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.345Medium
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.346Medium
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.347Medium
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.348Medium
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.349Medium
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.350Medium
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.351Medium
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.352Medium
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.353Medium
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.354Medium
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.
Q.355Medium
The diode equation for current through a p-n junction is I = I_s(e^(qV/kT) - 1). Here, I_s is:
Answer: A
I_s is the reverse saturation current, representing the leakage current when the junction is reverse biased. It depends on temperature and doping concentrations.
Q.356Medium
In a BJT operating in active mode, which of the following is correct?
Answer: B
In active mode, the BE junction is forward biased to inject carriers, while the BC junction is reverse biased to collect these carriers, ensuring transistor amplification.
Q.357Medium
In a MOSFET, the threshold voltage V_T increases with:
Answer: B
Higher substrate doping increases the surface potential needed to invert the channel, raising V_T. This is described by V_T = V_T0 + γ(√(2φ_f + V_SB) - √(2φ_f)).
Q.358Medium
In an n-channel MOSFET operating in saturation region, the drain current is approximately:
Answer: A
In saturation, the MOSFET acts as a voltage-controlled current source with I_D proportional to (V_GS - V_T)^2, independent of V_DS beyond saturation voltage.
Q.359Medium
The transconductance g_m of a MOSFET in saturation is:
Answer: A
Transconductance g_m = ∂I_D/∂V_GS measures how much the drain current changes with gate voltage, a key figure of merit in MOSFET amplifier design.
Q.360Medium
Which semiconductor material has the highest electron mobility at room temperature?
Answer: C
GaAs has electron mobility ~8500 cm²/Vs compared to Si (~1350 cm²/Vs), making it superior for high-frequency and high-speed applications.
