About this previous year paper
Course: B.Tech (Electronics / Electrical / Related Branches)
Semester: V Subject Code: KEC-056
Title: Advance Semiconductor Device Maximum Marks: 100
Duration: 3 Hours
Paper Type: Theory
Sections:
Section A: Short conceptual questions — 20 marks
Section B: Descriptive questions — 30 marks
Section C: Analytical / Device working — 50 marks
SECTION A — Short Answer Questions (10 × 2 = 20 Marks)
All questions are compulsory.
Define the term Crystal Structure:
The regular 3D arrangement of atoms or molecules in a solid, forming a repeating lattice pattern.
Describe the term Insulators:
Materials with a wide energy band gap (typically >3 eV) that prevent free movement of electrons, e.g., glass, rubber.
Explain the term Bipolar in BJTs:
Bipolar Junction Transistor (BJT) operation involves both electrons and holes as charge carriers, hence "bipolar".
What is a Non-Volatile Memory Device?
A memory that retains data even after power is removed, e.g., Flash, EEPROM, ROM.
What is Noise in Circuits?
Unwanted electrical signals that distort or interfere with the intended signal in a circuit.
Explain the term V-I Characteristics:
Relationship between voltage (V) across a device and the resulting current (I) through it — defines its electrical behavior.
Define Power Devices:
Semiconductor devices designed to handle large voltages and currents (e.g., SCRs, MOSFETs, IGBTs).
Elaborate the term Radioactive:
The spontaneous emission of energy or particles from unstable atomic nuclei (used in radiation-based sensors).
Describe the term Photoconductor:
A material whose conductivity increases when exposed to light (used in photoresistors).
Define the term Sensors:
Devices that detect physical quantities (temperature, pressure, light) and convert them into electrical signals.
SECTION B — Descriptive Questions (3 × 10 = 30 Marks)
Attempt any three questions.
(a) Energy Band Theory
Energy Band: Range of energy levels that electrons can occupy.
Comparison:
| Property | Conductor | Semiconductor | Insulator |
|---|---|---|---|
| Band Gap | 0 eV | 1.1 eV (Si) | >3 eV |
| Conduction Band | Overlaps with valence band | Separated by small gap | Separated by large gap |
| Example | Cu, Ag | Si, Ge | Glass, Quartz |
(b) Scaling and Short Channel Effects in FETs
As MOSFET dimensions shrink, electric field increases → short-channel effects:
Drain Induced Barrier Lowering (DIBL)
Velocity Saturation
Threshold voltage roll-off
Hot carrier effect
(c) Tunnel Diode
Working Principle: Quantum tunneling through a narrow depletion region in heavily doped p–n junction.
V–I Characteristics: Shows negative resistance region used in high-frequency oscillators.
(d) Light Emitting Diode (LED)
Working: Forward bias causes electron-hole recombination in a semiconductor, emitting photons.
V–I Characteristics: Nonlinear — current increases exponentially beyond threshold voltage.
Applications: Displays, indicators, optical communication.
(e) FPGA Architecture
Field Programmable Gate Array: Consists of configurable logic blocks (CLBs), interconnects, and I/O blocks.
Enables custom digital circuit design using hardware description languages (HDLs).
SECTION C — Analytical / Device Working (5 × 10 = 50 Marks)
Attempt one part from each question (Q3–Q7).
Q3. Metal–Semiconductor Devices or Varactor Diode
(a) Schottky Diode:
Formed between metal and n-type semiconductor.
V–I Curve: Shows fast switching and low forward voltage drop (~0.3 V).
Applications: High-speed rectifiers, RF detectors.
(b) Varactor Diode:
A diode with voltage-dependent junction capacitance.
V–I Curve: Reverse-biased operation.
Use: Voltage-controlled oscillator (VCO).
Q4. Heterojunction BJT or JFET
(a) Heterojunction BJT:
Combines two semiconductor materials with different band gaps (e.g., GaAs-AlGaAs).
Improves speed and gain for microwave applications.
(b) JFET (Junction Field Effect Transistor):
Current controlled by electric field across p–n junction. V–I Curve: Linear region followed by saturation region. Applications: Amplifiers, switches.
Q5. BARITT or TUNNETT Diode
(a) BARITT (Barrier Injection Transit-Time) Diode:
Generates microwave oscillations via injection and drift of minority carriers.
(b) TUNNETT Diode:
Uses avalanche breakdown and tunneling for negative resistance generation.
Used in microwave oscillators and amplifiers.
Q6. Thyristors or LASER Diode
(a) Thyristor (SCR):
Four-layer (PNPN) device with latching behavior. Used in AC power control, rectifiers.
(b) LASER Diode:
Light Amplification by Stimulated Emission of Radiation.
V–I Curve: Sharp threshold current beyond which coherent light is emitted.
Used in optical communication and barcode scanners.
Q7. CCD or Mechanical Sensors
(a) Charge-Coupled Device (CCD):
Converts light into electrical charge and shifts it for processing. Used in digital imaging and astronomy.
(b) Mechanical Sensors:
Measure parameters like pressure, strain, or displacement.
Based on piezoelectric, capacitive, or resistive principles.
Key Topics Covered
Semiconductor fundamentals & energy bands Tunnel, Varactor, LED, LASER diodes
BJTs, FETs, and heterojunction devices Power devices and thyristors
FPGA architecture Sensors and optoelectronic devices