Reverse bias widens the depletion region, allowing photogenerated carriers throughout this region to be swept out by the electric field, improving collection efficiency and response time.
Q.702Medium
In a solar cell, the maximum power output occurs when the operating point satisfies which condition?
Answer: C
Maximum power point (MPP) occurs at the knee of the I-V characteristic where the product V×I is maximum, typically at ~80% of Voc and ~90% of Isc.
Q.703Hard
The fill factor of a solar cell is defined as:
Answer: A
Fill Factor (FF) = Pmax/(Voc × Isc) = (Vm × Im)/(Voc × Isc). It indicates how close the I-V curve is to a rectangle, typically 0.7-0.85 for practical solar cells.
Q.704Hard
In an avalanche photodiode (APD), the internal gain (multiplication factor M) increases significantly due to:
Answer: B
APD operates in high reverse bias where impact ionization (collision ionization) produces secondary electron-hole pairs, creating avalanche multiplication and signal amplification internally.
Q.705Hard
The specific contact resistance of a metal-semiconductor junction is proportional to:
Answer: B
Contact resistance ρc ∝ exp(φB/kT)/Nc where φB is barrier height. Higher barrier leads to exponentially higher resistance following Thermionic emission theory.
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Q.706Medium
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.707Hard
Which combination of semiconductor parameters would result in the highest intrinsic carrier concentration at 300K?
Answer: B
ni = √(Nc·Nv)·exp(-Eg/2kT) where Nc, Nv depend on effective masses. Small Eg and high density of states both exponentially increase ni. GaAs has higher ni than Si due to smaller Eg.
Q.708Easy
In a p-n junction diode at thermal equilibrium, the direction of the electric field in the depletion region is:
Answer: B
At thermal equilibrium, the built-in electric field points from the p-region (positive) to n-region (negative) to oppose further diffusion of carriers.
Q.709Easy
The band gap energy of Germanium at room temperature (300K) is approximately:
Answer: A
Germanium has a band gap of ~0.66 eV at room temperature, making it a narrow band gap semiconductor used in infrared applications.
Q.710Easy
When a semiconductor is doped with donor atoms, the Fermi level shifts:
Answer: B
Donor doping creates an n-type semiconductor, shifting the Fermi level closer to the conduction band due to increased electron concentration.
Q.711Medium
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.712Medium
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.713Easy
A zener diode is used in reverse bias to:
Answer: B
Zener diodes are operated in reverse bias within the zener breakdown region to maintain constant voltage across them, useful for voltage regulation.
Q.714Medium
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.715Medium
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.
Q.716Easy
In a BJT (Bipolar Junction Transistor) in active mode, the base-emitter junction is:
Answer: B
In active mode, BE junction is forward biased (injects carriers) while BC junction is reverse biased (collects carriers).
Q.717Medium
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.718Medium
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.719Hard
In a tunnel diode, negative differential resistance occurs because:
Answer: C
In tunnel diode's NDR region, as voltage increases, tunneling current decreases (fewer states to tunnel into) while diffusion current increases, causing net decrease in total current.
Q.720Medium
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.
