(SEM III) THEORY EXAMINATION 2023-24 ELECTRONIC DEVICES

This question paper evaluates a student's understanding of semiconductor physics, energy band theory, electronic devices (PN junctions, transistors, MOSFETs), wave mechanics, and charge transport mechanisms. The exam is structured into three comprehensive sections, testing conceptual clarity, problem-solving skills, and deeper analytical reasoning.

SECTION A — Short Answer Questions (14 Marks)

Seven questions × 2 marks each

This section checks fundamental theoretical understanding of device physics.

Topics Covered:

1. Thermal Equilibrium Condition

Explains how carrier concentration, Fermi level, and recombination-generation balance remain constant in a semiconductor under equilibrium.

2. PN Junction Reverse Bias Energy Band Diagram

Requires sketching band bending, widened depletion region, increased barrier potential, and electric field direction.

3. Relation Between α and β in a Transistor

Given α = 0.98, calculate β using:
β = α / (1 – α)

4. Electroluminescence

Discusses the principle where recombination of electrons and holes emits photons (basis of LEDs).

5. de Broglie Principle of Duality

States the wave–particle duality of matter with λ = h/p.

6. Properties of MOS Capacitor

Key features: accumulation, depletion, inversion, oxide capacitance, and threshold voltage behavior.

7. Drift Current vs Diffusion Current

Differentiate motion due to electric field (drift) vs carrier concentration gradient (diffusion).

SECTION A checks conceptual clarity and precision in definitions.

SECTION B — Analytical Descriptive Questions (21 Marks)

Attempt any three × 7 marks each

This section requires explanation, diagrams, and reasoning.

1. Photoelectric Effect & Particle Nature of Light

Explain Einstein’s equation (hν = Φ + KE), threshold frequency, and how discrete photon energy proves particle nature.

2. Intrinsic vs Extrinsic Semiconductors

Discuss carrier concentration, Fermi level position, dopants (donor/acceptor), and I–type, N–type, P–type semiconductors.

3. Small-Signal Model of PN Junction Diode

Explain dynamic resistance, incremental changes, diode linearization under AC, and equivalent circuit.

4. Stability Factor in Transistor Biasing

Define stability factor S, its importance, why unstable biasing causes thermal runaway, and methods to improve bias stability.

5. C-V Characteristics of MOS Transistor

Plot and explain accumulation, depletion, and inversion regions; oxide capacitance; threshold voltage.

SECTION C — Long Answer / Analytical Questions (35 Marks)

One question from each part × 7 marks

3. Quantum Mechanics & Band Theory

Option A – Schrödinger Equation in Infinite Potential Well

Explain quantized energy levels, wave functions, effect of well width on energy, and confinement.

Option B – Allowed & Forbidden Bands (Kronig–Penney Model)

Illustrate formation of energy bands, band gaps, periodic potential, and qualitative/rigorous explanation from model results.

4. Doping & Carrier Transport

Option A – Doping and Energy Band Gap Changes

Explain donor/acceptor levels, shift in Fermi energy, narrowing/widening of band gap, and effect on conductivity.

Option B – Continuity Equation

Derive using drift and diffusion currents, charge conservation, and recombination-generation processes.

5. Carrier Transport, Diode Equation & Einstein Relation

Option A – Einstein Relation

Derive D/μ = kT/q and explain physical importance in diffusion and mobility.

Option B – PN Junction Diode I-V Relation

Derive Shockley diode equation:
I = I₀ (e^{qV/ηkT} – 1)
Discuss forward & reverse bias behavior.

6. Transistor Modeling & Biasing

Option A – Ebers–Moll Model for PNP Transistor

Explain large-signal model, forward/reverse biasing, current equations, and equivalent circuit.

Option B – Biasing Schemes & Voltage Divider Bias

List all schemes:
• Fixed bias
• Collector-to-base bias
• Voltage-divider bias
• Emitter bias
Explain voltage divider bias with diagram, derivation of stability, and operating point selection.

7. JFET & MOSFET Characteristics

Option A – JFET Biasing Schemes

Explain various biasing types:
• Self-bias
• Voltage-divider bias
• Fixed-bias
• Current-source bias

Option B – Enhancement-Type P-Channel MOSFET

Discuss structure, operation (VSG > Vt), channel formation, transfer and drain characteristics with diagram.