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.742Medium
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.743Hard
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.744Hard
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.745Medium
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.
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Q.746Easy
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.747Easy
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).
Q.748Easy
In a p-type semiconductor, the majority carriers are:
Answer: B
P-type semiconductors are doped with acceptor impurities which create holes as majority carriers. Electrons become minority carriers in p-type material.
Q.749Easy
When a p-n junction is reverse biased, the width of the depletion region:
Answer: C
Reverse bias strengthens the electric field across the junction, causing the depletion region to expand as more charge carriers are pulled away from the junction.
Q.750Medium
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.751Medium
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.752Easy
The current gain β (beta) of a BJT is defined as:
Answer: A
The current gain β is the ratio of collector current to base current (I_c/I_b). For silicon transistors, typical β values range from 50 to 500.
Q.753Medium
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.754Medium
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.755Medium
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.756Medium
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.757Medium
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.758Medium
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.759Hard
In a Zener diode, breakdown voltage is primarily determined by:
Answer: B
Higher doping concentrations create stronger electric fields at lower voltages, reducing Zener breakdown voltage. This allows designing Zeners with specific breakdown voltages (3.3V, 5.1V, etc.).
Q.760Hard
In a tunnel diode, the negative resistance region occurs due to:
Answer: B
At low forward bias, electrons tunnel through the narrow bandgap, creating peak current. As voltage increases, direct band-to-band current dominates, causing current to decrease with voltage (negative differential resistance).
