In compensated silicon with ND = 2×10¹⁶ and NA = 5×10¹⁵ cm⁻³, what is the net donor concentration?
A 1.5×10¹⁶ cm⁻³
B 2.5×10¹⁶ cm⁻³
C 2.5×10¹⁵ cm⁻³
D 1.0×10¹⁷ cm⁻³
Answer: A. 1.5×10¹⁶ cm⁻³ For fully ionized donors and acceptors, net n-type doping is N_D − N_A = 1.5×10¹⁶ cm⁻³.
Adding donors to intrinsic silicon shifts the Fermi level toward which band?
A Valence band
B Conduction band
C Bandgap middle
D Below the valence band
Answer: B. Conduction band Donor doping raises electron concentration and shifts the Fermi level toward the conduction band.
Under illumination far from equilibrium, which levels describe electron and hole populations separately?
A Quasi-Fermi levels
B Vacuum levels
C Donor levels
D Phonon levels
Answer: A. Quasi-Fermi levels Nonequilibrium carrier populations are described by separate electron and hole quasi-Fermi levels.
Raising acceptor doping at fixed temperature changes built-in potential how in a nondegenerate junction?
A It becomes zero
B It is unchanged
C It increases
D It decreases
Answer: C. It increases Charge neutrality gives N_D x_n = N_A x_p, so x_n/x_p = N_A/N_D.
Given NA = 4ND in an abrupt p–n junction, which side contains four times the depletion width?
A equal widths
B n side fourfold
C zero-width side
D p side twice
Answer: B. n side fourfold Charge neutrality gives N_A x_p = N_D x_n. With N_A = 4N_D, x_n = 4x_p, so the n side has four times the width.
As reverse bias approaches breakdown, what happens to the junction’s peak electric field?
A It reverses permanently
B It stays zero
C It vanishes
D It increases
Answer: D. It increases Heavy doping makes the depletion region thin and the electric field strong, favoring band-to-band Zener tunneling.
During low-level injection in p-type silicon, what relation is assumed?
A p₀ = 0
B n₀ = p₀
C Δn ≪ p₀
D Δn ≫ p₀
Answer: C. Δn ≪ p₀ Low-level injection assumes the excess minority-carrier concentration remains small compared with the equilibrium majority-carrier concentration.
How does diode reverse saturation current scale with junction area at fixed conditions?
A It is area-independent
B It falls with area
C It becomes zero
D It rises with area
Answer: D. It rises with area For a uniform junction at fixed conditions, saturation current is proportional to junction area.
At high forward current, what effect does diode series resistance have on terminal voltage?
A Cancels the junction drop
B Makes voltage zero
C Adds an ohmic drop
D Removes stored charge
Answer: C. Adds an ohmic drop At high current, the series resistance contributes an additional IR drop, so terminal voltage departs from the ideal exponential junction relation.
In a PIN photodiode, a wide intrinsic layer mainly reduces which parasitic quantity?
A Carrier lifetime
B Bandgap voltage
C Junction capacitance
D Photon energy
Answer: C. Junction capacitance A wide depletion region increases charge separation and generally lowers junction capacitance, aiding fast photodetection.
With a negative gate bias on a p-type MOS capacitor, which surface condition forms?
A Inversion
B Accumulation
C Depletion
D Avalanche
Answer: B. Accumulation A negative gate attracts holes, the majority carriers in a p-type substrate, producing accumulation.
For a p-type MOS substrate, strong inversion begins near which surface-potential magnitude?
A |ψs| = 4kT/q
B |ψs| = 2φF
C |ψs| = 0
D |ψs| = φF/2
Answer: B. |ψs| = 2φF The conventional strong-inversion criterion for a p-type substrate is a surface potential magnitude near 2φ_F.
During a high-frequency MOS capacitor C–V test, why may inversion capacitance stay near its minimum?
A Minority carriers cannot respond quickly
B Gate voltage cannot change
C Oxide becomes a conductor
D Majority carriers vanish
Answer: A. Minority carriers cannot respond quickly At high frequency, minority carriers cannot be generated or respond quickly enough to follow the small-signal voltage.
Below threshold, which transport mechanism dominates current in a long-channel MOSFET?
A Diffusion
B Avalanche
C Impact ionization
D Thermionic emission only
Answer: A. Diffusion In weak inversion, long-channel MOSFET current is dominated by diffusion and varies exponentially with gate voltage.
Punch-through in a short-channel MOSFET occurs when what happens?
A Gate oxide becomes thicker
B The channel turns into metal
C The bandgap closes
D Depletion regions merge
Answer: D. Depletion regions merge Punch-through occurs when source and drain depletion regions approach or merge, weakening gate control of the channel barrier.
Shorter channel length at low drain bias produces which threshold-voltage trend?
A Drain current becomes zero
B Threshold rises with channel length
C Threshold falls as channel shortens
D Gate capacitance disappears
Answer: C. Threshold falls as channel shortens Shorter channels can reduce threshold voltage because source and drain electrostatics increasingly influence the channel.
Velocity saturation limits MOSFET current when what happens?
A Velocity saturation
B Photovoltaic action
C Base-width modulation
D Dopant compensation
Answer: A. Velocity saturation At high fields, carrier velocity approaches saturation, limiting the current increase predicted by a low-field mobility model.
Why can a high-k gate dielectric reduce tunneling leakage at equal capacitance?
A Raises leakage current
B Lowers capacitance to zero
C Allows thicker dielectric
D Removes the gate
Answer: D. Removes the gate A high-k material can provide the needed capacitance with a thicker physical dielectric, reducing direct tunneling.
What is a key electrostatic advantage of a FinFET over a planar single-gate device?
A No depletion region
B One-sided carrier injection
C Gate control from multiple sides
D Larger base current
Answer: C. Gate control from multiple sides A FinFET’s gate surrounds multiple channel surfaces, improving electrostatic control in scaled devices.
Long-channel saturation gives what approximation for nMOS transconductance?
A gm ≈ 1/ID
B gm ≈ VOV/ID²
C gm ≈ IDVOV
D gm ≈ 2ID/VOV
Answer: D. gm ≈ 2ID/VOV For a long-channel MOSFET in square-law saturation, g_m ≈ 2I_D/V_OV.
Population inversion at laser threshold enables which process to dominate?
A Population inversion
B Depletion collapse
C Bandgap closure
D Carrier freeze-out
Answer: A. Population inversion Laser action begins when stimulated emission can exceed absorption, which requires population inversion in the active medium.
With VA = 50 V and IC = 1 mA, what is BJT output resistance ro approximately?
A 500 Ω
B 5 kΩ
C 500 kΩ
D 50 kΩ
Answer: D. 50 kΩ Using r_o ≈ V_A/I_C gives 50 V / 1 mA = 50 kΩ.
During reverse recovery, what charge mainly must leave a previously forward-conducting p–n diode?
A Stored minority-carrier charge
B Gate charge
C Ionized dopant charge
D Majority lattice charge
Answer: A. Stored minority-carrier charge Forward conduction stores minority carriers; reverse current removes this stored charge before blocking is restored.
Which layer arrangement improves carrier confinement in many LED heterostructures?
A Intrinsic silicon in copper
B Narrow-gap active layer
C Metal between two oxides
D One wide depletion layer
Answer: B. Narrow-gap active layer A lower-bandgap active region between higher-bandgap layers confines carriers and supports efficient radiative recombination.
For band-to-band emission, what mainly sets the approximate photon energy?
A Junction width only
B Semiconductor bandgap
C Series resistance
D Dopant atomic mass
Answer: B. Semiconductor bandgap A band-to-band photon has energy approximately equal to the semiconductor bandgap, with wavelength inversely related to energy.