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.
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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.
