Semiconductor Electronics Class 12 Notes | CBSE Physics Chapter 14

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)

---

Previous: Ch 13 — Nuclei
First Chapter: Ch 1 — Electric Charges and Fields