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.
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.42Medium
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.43Medium
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.44Medium
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.45Medium
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.46Medium
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.
Q.47Medium
In a light-emitting diode (LED), the color of emitted light depends primarily on:
Answer: B
The wavelength of emitted light λ = hc/E_g, where E_g is the bandgap. Different materials (GaAs, GaN, AlGaAs) emit different colors based on their bandgap energy.
Q.48Medium
A solar cell operates as a p-n junction under illumination. The maximum power output occurs at:
Answer: C
Maximum power point (MPP) occurs where P = V × I is maximum, typically at ~80% of V_oc and ~80% of I_sc, determined by the fill factor.
Q.49Medium
In a p-n junction diode, the depletion width is primarily determined by which of the following factors?
Answer: A
The depletion width W = √(2εε₀(V_bi + V_R)/(eN_D N_A/(N_D + N_A))) depends on applied voltage, doping concentrations, and built-in potential. Both doping levels and reverse bias voltage are critical factors.
Q.50Medium
A photodiode is reverse-biased to operate in photoconductive mode. What is the primary advantage of this configuration over photovoltaic mode?
Answer: B
In photoconductive (reverse-bias) mode, the depletion region widens significantly, reducing junction capacitance and transit time, resulting in faster response (MHz to GHz range) and shot noise is suppressed due to the reverse field.
