Semiconductor Devices — Previous Year Questions (Class 12 Physics)
Semiconductors are the foundation of modern electronics. Understanding p-n junctions, transistors, and logic gates is essential for electrical engineering
TL;DR: Semiconductors are the foundation of modern electronics. Understanding p-n junctions, transistors, and logic gates is essential for electrical enginee…
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Semiconductors are the foundation of modern electronics. Understanding p-n junctions, transistors, and logic gates is essential for electrical engineering
Semiconductor Devices — Previous Year Questions with Solutions
Q (2023, 3 marks): Explain the structure and working of a p-n junction diode. What happens at the depletion region?
Answer: Structure:
- p-type semiconductor (holes as majority carriers) joined with n-type (electrons as majority carriers).
- At the junction, diffusion occurs: electrons move from n to p, holes from p to n.
Depletion region:
1. Electrons leaving n-type leave behind positive ions; holes leaving p-type leave negative ions.
2. This creates an electric field pointing from n to p (built-in potential).
3. The region becomes depleted of mobile charges (~10^-7 m thick).
4. This field opposes further diffusion, reaching equilibrium.
Biasing:
Forward bias: Positive on p-side reduces depletion width, allows large current.
Reverse bias: Negative on p-side increases depletion width, blocks current (except small leakage).
I-V characteristic: Exponential increase in forward current, minimal reverse current until breakdown.
Q (2022, 3 marks): Describe the construction and working of a bipolar junction transistor (BJT) in the active region.
Answer: BJT construction:
- Two back-to-back p-n junctions: p-n-p (PNP) or n-p-n (NPN)
- Three terminals: Emitter (E), Base (B), Collector (C)
- For NPN: very thin p-type base between n-type emitter and collector
Working in active region:
1. Emitter-Base junction: Forward biased (small voltage, large current)
2. Collector-Base junction: Reverse biased
3. Electrons from emitter diffuse through thin base to collector (most reach collector)
4. Base current IB controls collector current IC
5. Current gain: β = IC/IB (typically 50-100)
6. Emitter current: IE = IC + IB
Applications:
- Amplification: Small base current controls large collector current
- Switching: Can be turned on/off rapidly
- Logic circuits: Forms the basis of digital electronics
Q (2023, 3 marks): Explain the working of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).
Answer: MOSFET structure:
- n-type channel between source (S) and drain (D)
- Insulated gate (G) over the channel (separated by SiO2)
- p-type substrate beneath
Working:
1. Gate voltage VG controls the channel
2. When VG > threshold voltage:
- Electric field attracts electrons from substrate
- Thin electron channel forms between source and drain
- Current ID flows from drain to source
3. When VG ≤ threshold:
- No channel forms, ID ≈ 0 (off state)
4. Drain current: ID depends on VG (in saturation region, ID is nearly independent of VDS)
Advantages over BJT:
- High input impedance (no gate current flows)
- Voltage-controlled (vs. current-controlled in BJT)
- Faster switching
- Lower power consumption
- Easily integrated in ICs
Types: NMOS (n-channel), PMOS (p-channel), CMOS (complementary)
Q (2022, 3 marks): What are logic gates? Draw the truth table and symbol for AND, OR, and NOT gates.
Answer: Logic gates: Devices that perform basic Boolean operations on binary inputs (0 or 1).
AND gate:
Symbol: D shape with flat input side
Truth table:
A B | Output
0 0 | 0
0 1 | 0
1 0 | 0
1 1 | 1
Output = A AND B (both must be 1)
OR gate:
Symbol: D shape with curved input side
Truth table:
A B | Output
0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 1
Output = A OR B (at least one must be 1)
NOT gate (Inverter):
Symbol: Triangle with bubble at output
Truth table:
A | Output
0 | 1
1 | 0
Output = NOT A (inverts the input)
Q (2023, 2 marks): Explain the difference between analog and digital circuits. Give examples of each.
Answer: Analog circuits:
- Process continuous signals with infinite intermediate values.
- Signal can take any value within a range (0-10V, for example).
- Examples: Audio amplifiers, radio receivers, oscilloscopes, thermometers
- Advantages: Good signal resolution, simpler for some applications
- Disadvantages: Sensitive to noise, component tolerances affect performance
Digital circuits:
- Process discrete signals with only two values (0 and 1, or low and high voltage).
- Signal is binary: either 0 or 1 (e.g., 0-0.8V = 0, 2-5V = 1)
- Examples: Computers, calculators, digital watches, microcontrollers
- Advantages: Noise immunity, accurate, easy to design and scale, perfect for computing
- Disadvantages: Reduced resolution, sampling rate limitations
Q (2021, 3 marks): Design a NOT gate using a transistor. Explain its operation with a truth table.
Answer: NOT gate using NPN transistor:
Circuit:
- VCC (positive supply) connected to collector through resistor RC
- Input signal A at base through resistor RB
- Emitter grounded
- Output taken from collector
Operation:
When A = 1 (high input, ~5V):
- Base current flows, transistor saturates
- VCE ≈ 0.2V (low), Output ≈ 0 (Logic 0)
When A = 0 (low input, ~0V):
- No base current, transistor cuts off
- VCE ≈ VCC (high), Output ≈ VCC (Logic 1)
Truth table:
Input A | Output
0 | 1
1 | 0
This inverts the input, realizing the NOT gate function.
Frequently Asked Questions
What is the difference between n-type and p-type semiconductors?
n-type (electron-type): Doped with donor impurities (Group 5 elements like Phosphorus). Electrons are majority carriers, holes are minority carriers. Fermi level lies closer to conduction band. p-type (hole-type): Doped with acceptor impurities (Group 3 elements like Boron). Holes are majority carriers, electrons are minority carriers. Fermi level lies closer to valence band.
Why is the base of a transistor made very thin?
The base is made very thin (~1 micrometer) so that most electrons injected from the emitter reach the collector before recombining with holes. If the base were thicker, more electrons would recombine in the base, reducing the current gain (β). The thin base ensures high current amplification.
More Class 12 Physics PYQs
- Current Electricity
- Ray Optics
- Electrostatics
- Moving Charges and Magnetism
- Electromagnetic Induction
- Alternating Current
🤖 Stuck on any of these? Ask Syllab's free AI Tutor to explain step by step →