Semiconductor Electronics is Chapter 14 of CBSE Class 12 Physics. This chapter covers the basics of semiconductors, p-n junctions, diodes, transistors, and logic gates. Understanding semiconductors is essential as they are the foundation of all modern electronic devices — from mobile phones to computers.
This chapter carries 5–7 marks. P-N junction diode, transistor as amplifier/switch, and logic gates are the most tested topics.
Key Concepts
1. Energy Bands in Solids
| Type | Band Gap | Examples |
|---|---|---|
| Conductor | No gap (bands overlap) | Copper, silver, aluminium |
| Semiconductor | Small gap (~1 eV) | Silicon (1.1 eV), Germanium (0.7 eV) |
| Insulator | Large gap (> 3 eV) | Diamond, rubber, glass |
2. Intrinsic and Extrinsic Semiconductors
Intrinsic: Pure semiconductor (equal electrons and holes). Conductivity is low.
Extrinsic: Doped with impurities to increase conductivity.
| Type | Dopant | Majority Carriers | Example |
|---|---|---|---|
| n-type | Pentavalent (P, As, Sb) | Electrons | Si doped with As |
| p-type | Trivalent (B, Al, Ga, In) | Holes | Si doped with B |
3. P-N Junction Diode
When p-type and n-type are joined, a depletion region forms at the junction (thin region with no free carriers).
- Forward bias: P connected to +, N to −. Current flows (barrier reduced). Diode conducts above ~0.7 V (Si) or ~0.3 V (Ge).
- Reverse bias: P to −, N to +. Very small reverse current (barrier increased). Diode blocks.
Applications of Diode
- Rectifier: Converts AC to DC (half-wave or full-wave)
- Zener diode: Voltage regulator (maintains constant output voltage)
- LED: Emits light when forward biased
- Photodiode: Generates current when light falls on it (reverse biased)
- Solar cell: Converts light to electricity (no external bias needed)
4. Transistor
A transistor has three layers (NPN or PNP) and three terminals: Emitter (E), Base (B), Collector (C).
Current relation: IE = IB + IC (emitter current = base + collector)
Current gain: β = IC/IB (typically 20–200)
Transistor as Amplifier (Common Emitter)
Small change in base current → large change in collector current.
Voltage gain: Av = −βRL/Ri
Input: base-emitter circuit; Output: collector-emitter circuit.
Transistor as Switch
Cutoff region: both junctions reverse biased → transistor OFF (switch open)
Saturation region: both junctions forward biased → transistor ON (switch closed)
5. Logic Gates
| Gate | Symbol | Boolean | Output |
|---|---|---|---|
| AND | A · B | Y = A·B | 1 only if both inputs are 1 |
| OR | A + B | Y = A+B | 1 if any input is 1 |
| NOT | Ā | Y = Ā | Inverts input |
| NAND | (A·B)̄ | Y = (A·B)̄ | 0 only if both inputs are 1 |
| NOR | (A+B)̄ | Y = (A+B)̄ | 1 only if both inputs are 0 |
NAND and NOR are universal gates — any logic function can be built using only NAND or only NOR gates.
Solved Examples
Example 1
In a transistor, IB = 50 μA and IC = 5 mA. Find IE and β.
Answer: IE = IB + IC = 0.05 + 5 = 5.05 mA. β = IC/IB = 5/0.05 = 100.
Example 2
A Zener diode has breakdown voltage 5 V. Input voltage varies from 7–12 V with 500 Ω series resistance. Find current range.
Answer: I = (V_in − V_z)/R. Min: (7−5)/500 = 4 mA. Max: (12−5)/500 = 14 mA. Output stays at 5 V.
Quick Revision Points
- Semiconductor band gap: Si = 1.1 eV, Ge = 0.7 eV
- n-type: pentavalent dopant (majority: electrons); p-type: trivalent (majority: holes)
- Diode: forward bias → conducts; reverse bias → blocks
- Zener: voltage regulator; LED: light emission; Solar cell: light → electricity
- Transistor: IE = IB + IC; β = IC/IB; amplifier (CE mode): Av = −βRL/Ri
- Logic gates: AND, OR, NOT, NAND (universal), NOR (universal)
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First Chapter: Ch 1 — Electric Charges and Fields
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