Question 1

Assertion (A): A p-n junction has high resistance in the reverse direction. Reason (R): Applying reverse bias increases the width of the depletion layer.

Accepted Answer

Introduction / Context: Reverse-bias characteristics of p-n junctions underpin rectification and blocking behavior in diodes. Understanding depletion layer modulation explains the high reverse resistance. Given Data / Assumptions: Junction: p-n silicon diode. Bias: reverse voltage applied. Concept / Approach: Reverse bias increases built-in potential, widens the depletion region, and lowers carrier injection, so only a tiny leakage current flows. This corresponds to a very large effective resistance. Step-by-Step Solution: Step 1: Reverse bias raises the barrier.Step 2: Depletion region width increases, reducing majority-carrier flow.Step 3: Leakage is limited to minority carriers; current is very small.Step 4: Hence reverse resistance is high and R explains A. Verification / Alternative check: I-V characteristics of diodes show microampere to nanoampere reverse currents until breakdown, corroborating high resistance. Why Other Options Are Wrong: B: R directly explains A, not merely co-occurring.C/D/E: Each contradicts standard diode theory. Common Pitfalls: Assuming zero current under reverse bias; there is always small leakage until breakdown. Final Answer: Both A and R are true and R is correct explanation of A

Question 2

A semiconductor has minority-carrier lifetime τ = 100 μs and diffusion constant D = 100 cm^2/s. Compute the diffusion length L.

Accepted Answer

Introduction / Context: Diffusion length is a key transport parameter indicating the average distance a minority carrier travels before recombining. It is widely used in device design and analysis (e.g., BJTs, solar cells). Given Data / Assumptions: Minority-carrier lifetime τ = 100 μs = 1.0e-4 s. Diffusion constant D = 100 cm^2/s. Isotropic, low-level injection conditions. Concept / Approach: The diffusion length L is defined by L = sqrt(D * τ). This comes from the solution of the diffusion-recombination equation for minority carriers. Step-by-Step Solution: Step 1: Write formula: L = sqrt(D * τ).Step 2: Substitute: D = 100 cm^2/s, τ = 1.0e-4 s.Step 3: Compute product: D * τ = 100 * 1.0e-4 = 1.0e-2.Step 4: Take square root: L = sqrt(1.0e-2) = 0.1 cm. Verification / Alternative check: Order-of-magnitude check: If D is large and τ is modest, L is typically in the sub-millimeter to centimeter range; 0.1 cm is reasonable. Why Other Options Are Wrong: 0.01 cm and 0.0141 cm: Result from incorrect arithmetic or unit mishandling.1 cm and 0.316 cm: Overestimates due to using D or τ alone or taking incorrect square roots. Common Pitfalls: Mixing SI and cgs units; forgetting to convert μs to seconds; taking sqrt(D/τ) instead of sqrt(D*τ). Final Answer: 0.1 cm

Question 3

Compared with an ordinary p-n diode, a Schottky diode exhibits which combination of reverse saturation current and cut-in voltage?

Accepted Answer

Introduction / Context: Schottky diodes use a metal–semiconductor junction. Their transport and I-V characteristics differ from p-n junctions, offering low forward drop and fast switching at the expense of higher leakage. Given Data / Assumptions: Device types: Schottky diode vs p-n diode. Properties compared: reverse saturation current (leakage) and forward cut-in voltage. Concept / Approach: Due to the metal–semiconductor barrier and majority-carrier conduction, Schottky diodes typically have low forward voltage (about 0.2–0.4 V) and relatively high reverse leakage compared with silicon p-n diodes (about 0.6–0.7 V forward drop, lower leakage). Step-by-Step Solution: Step 1: Identify forward behavior: Schottky has lower cut-in voltage.Step 2: Identify reverse behavior: Schottky has higher leakage (reverse saturation current) at a given reverse voltage.Step 3: Select the option reflecting both facts. Verification / Alternative check: Datasheets and application notes consistently show Schottky forward drop lower than silicon p-n and leakage higher, especially at elevated temperatures. Why Other Options Are Wrong: A: Incomplete; misses forward-voltage aspect.B: Incorrect; forward drop is not higher.D/E: Misstate one or both trends. Common Pitfalls: Confusing Schottky with germanium p-n diodes; conflating forward drop with conduction loss only and ignoring leakage impact on efficiency. Final Answer: has higher reverse saturation current and lower cut in voltage

Question 4

In a p-type semiconductor where the Fermi level is initially 0.3 eV above the valence band, what happens to the Fermi level when acceptor concentration is increased?

Accepted Answer

Introduction / Context: The Fermi level indicates the chemical potential for electrons. In doped semiconductors, its position shifts with dopant concentration and type, affecting carrier densities and device behavior. Given Data / Assumptions: Material: p-type semiconductor. Initial EF position: 0.3 eV above Ev (valence band edge). Action: Increase acceptor concentration. Concept / Approach: Increasing acceptor doping raises hole concentration, pulling EF closer to the valence band. Thus EF moves downward toward Ev (i.e., decreases the separation EF − Ev). Step-by-Step Solution: Step 1: Recognize trend: stronger p-type doping → EF approaches Ev.Step 2: From 0.3 eV above Ev, a higher acceptor density reduces this offset.Step 3: Among the choices, 0.28 eV above Ev reflects a modest shift closer to Ev, consistent with increased acceptor density. Verification / Alternative check: Qualitative band diagram analysis confirms EF movement toward Ev with higher p-type doping; quantitative shift depends on temperature and exact dopant concentration. Why Other Options Are Wrong: 0.5 eV above Ev: Moves away from Ev, opposite of expected behavior.0.1 eV above Ev: Could occur for a much larger increase but is not the best modest-step representation.Below Ev: EF cannot lie inside the valence band in equilibrium for a semiconductor (without degeneracy effects not implied here).Toward midgap regardless of doping: Incorrect trend for p-type doping. Common Pitfalls: Confusing the directions EF moves for n-type vs p-type; assuming EF crosses into bands under standard nondegenerate conditions. Final Answer: 0.28 eV above valence band

Question 5

Which semiconductor material is most commonly used for modern electronic devices and integrated circuits?

Accepted Answer

Introduction / Context: Material choice in semiconductor manufacturing balances cost, availability, native oxide quality, and device performance. Silicon dominates mainstream electronics and IC fabrication. Given Data / Assumptions: Focus: ubiquity and practicality for large-scale manufacturing. Comparison set: Si, Ge, SiGe alloys, others. Concept / Approach: Silicon offers an excellent native oxide (SiO2) for MOS processes, plentiful raw material, mature processing infrastructure, and good thermal properties. Germanium and compound semiconductors have niches but lack silicon’s overall ecosystem dominance. Step-by-Step Solution: Step 1: Evaluate device fabrication needs (oxide quality, yield, scalability).Step 2: Silicon’s native oxide enables stable MOSFET gates and isolation.Step 3: Global supply chains and cost favor silicon for most applications.Step 4: Therefore the correct choice is silicon. Verification / Alternative check: Industry statistics and foundry capacities overwhelmingly use Si; specialized domains (RF, optoelectronics) may use GaAs, InP, or SiGe, but mainstream is Si. Why Other Options Are Wrong: Germanium: Higher mobility but inferior oxide and thermal robustness.SiGe: Important for heterojunction devices but not the primary substrate in mass production.None: Contradicts industry practice.GaAs: Great for high-frequency/optoelectronics but not most general ICs. Common Pitfalls: Assuming material superiority in one metric determines dominance; manufacturing ecosystem matters greatly. Final Answer: silicon

Question 6

Photoelectric effect statements: which is correct regarding emitted electrons and light parameters?

Accepted Answer

Introduction / Context: The photoelectric effect links photon energy to the kinetic energy of emitted electrons from a metal surface, described by Einstein's equation. It is foundational for modern quantum theory and optoelectronics. Given Data / Assumptions: Metallic photocathode in vacuum. Incident light characterized by frequency f (or wavelength λ). Light intensity affects photon flux, not photon energy. Concept / Approach: Electron kinetic energy: KEmax = h f − φ, where φ is work function. Thus higher frequency (shorter wavelength) photons impart higher maximum kinetic energy to emitted electrons. Intensity mainly affects the number of emitted electrons (emission rate), not their maximum energy. Step-by-Step Solution: Step 1: Write Einstein relation: KEmax = h f − φ.Step 2: Increasing frequency (decreasing λ) increases KEmax and hence maximum velocity vmax.Step 3: Intensity changes photon count and thus emission rate, not KEmax.Step 4: Therefore, the correct statement is that maximum velocity increases with decreasing wavelength. Verification / Alternative check: Classical experiments: Stopping potential rises with frequency, independent of intensity, confirming dependence of KE on frequency. Why Other Options Are Wrong: A/E: Velocity (energy) depends on frequency, not intensity.B: Emission rate is directly, not inversely, proportional to intensity (if above threshold).D: Emission releases electrons only; metal does not emit free holes into vacuum. Common Pitfalls: Confusing photon energy (frequency) with photon flux (intensity); mixing photoelectric emission with photoconductive effects in solids. Final Answer: Maximum velocity of electron increases with decreasing wavelength

Question 7

A transistor has small-signal current gain α_ac = 0.98 in common-base mode. What is the corresponding β_ac in common-emitter mode?

Accepted Answer

Introduction / Context: Common-base (CB) and common-emitter (CE) current gains (α and β) are related for a bipolar junction transistor. Converting between them is routine in circuit analysis and device modeling. Given Data / Assumptions: α_ac = 0.98 (CB current gain). Small-signal, linear operating region. Concept / Approach: The exact relationship between CE and CB current gains is β = α / (1 − α). This follows from current relations in a BJT: IE = IC + IB, with α = IC / IE and β = IC / IB. Step-by-Step Solution: Step 1: Use β = α / (1 − α).Step 2: Substitute α = 0.98.Step 3: Compute denominator: 1 − 0.98 = 0.02.Step 4: β = 0.98 / 0.02 = 49. Verification / Alternative check: Cross-check with inverse: α = β / (β + 1) = 49 / 50 ≈ 0.98, consistent. Why Other Options Are Wrong: 51 or 47 or 45: Do not satisfy α = β / (β + 1) near 0.98.0.98: That is α, not β. Common Pitfalls: Using β ≈ α / (1 − α) but miscomputing the small denominator; forgetting that small α error produces large β change. Final Answer: 49

Electronic Devices and Circuits Questions