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.
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.62Easy
In a p-n junction at equilibrium, the potential difference across the junction is called:
Answer: B
The built-in potential (V₀) develops naturally across a p-n junction due to diffusion of charge carriers. For silicon at 300 K, V₀ ≈ 0.7 V, independent of external voltage.
Q.63Medium
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.64Medium
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.65Medium
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.66Hard
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.67Hard
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.68Medium
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.69Hard
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.70Medium
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.71Hard
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.72Hard
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.73Medium
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.74Medium
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.75Medium
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.76Hard
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.77Hard
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.78Medium
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.79Easy
In an intrinsic semiconductor at room temperature, what is the relationship between electron concentration (n_e) and hole concentration (n_h)?
Answer: B
In an intrinsic semiconductor, every electron-hole pair is generated together, so the concentration of free electrons equals the concentration of holes, both equal to the intrinsic carrier concentration n_i.
Q.80Easy
The forbidden energy gap (E_g) of germanium at 300K is approximately:
Answer: B
Germanium has a bandgap of approximately 0.66 eV at room temperature (300K), making it a narrow bandgap semiconductor compared to silicon (1.1 eV).
