In the Einstein relation for semiconductors, the ratio of diffusion coefficient to mobility (D/μ) equals:
Answer: B
Einstein relation: D/μ = kT/e. At room temperature (300K), this thermal voltage ≈ 26 mV. This fundamental relation connects drift and diffusion processes in semiconductors.
Q.22Medium
The temperature coefficient of resistance for semiconductors is:
Answer: B
Semiconductors have negative temperature coefficient because increased temperature increases intrinsic carrier concentration, decreasing resistance.
Q.23Medium
In a metal-semiconductor contact, if the metal work function is greater than the semiconductor work function, a __________ barrier is formed:
Answer: A
When metal work function > semiconductor work function, electrons transfer from semiconductor to metal, creating a Schottky barrier.
Q.24Medium
In an LED, the wavelength of emitted light depends on:
Answer: B
The energy of emitted photons equals the band gap energy (E = Eg = hc/λ), determining the color/wavelength of LED light.
Q.25Medium
The Hall coefficient for a p-type semiconductor is:
Answer: A
Hall coefficient is positive for p-type (hole conduction) and negative for n-type (electron conduction), based on the sign of majority carriers.
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Q.26Medium
The transconductance (gm) of a MOSFET increases with:
Answer: A
Transconductance gm ∝ W/L (width-to-length ratio), so decreasing channel length L increases transconductance.
Q.27Medium
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.28Medium
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.29Medium
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.30Medium
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.31Medium
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.32Medium
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.33Medium
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.34Medium
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.35Medium
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.36Medium
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.37Medium
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.38Medium
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.39Medium
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.40Medium
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.
