Class 12 Physics Notes

Semiconductor Electronics Class 12 Notes

Complete, exam-ready notes on semiconductor electronics: energy bands and doping, the p-n junction diode and its biasing, rectifiers, LED, photodiode and solar cell, transistors as amplifiers, and logic gates — written for CBSE boards, JEE and NEET revision.

Class12SubjectPhysicsCoversCBSE · JEE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What is semiconductor electronics in one line?

Semiconductor electronics harnesses the controlled conductivity of doped silicon and germanium to make diodes, transistors, rectifiers and logic gates — the building blocks of all modern digital devices.

Energy Bands and Doping

Energy bands

In solids, atomic levels broaden into bands: the valence band (filled) and the conduction band (empty or partial). A material is a conductor, semiconductor or insulator depending on the band gap — for silicon about 1.1 eV and germanium about 0.7 eV.

Intrinsic semiconductors (pure Si, Ge) have equal numbers of electrons and holes.
n-type doping adds pentavalent atoms (P, As, Sb): electrons become majority carriers.
p-type doping adds trivalent atoms (B, Al, In): holes become majority carriers.
At ordinary temperature the conductivity of a semiconductor rises sharply — unlike metals, which conduct better when cooler.

Doping, not new charge

Doped crystals stay electrically neutral overall. Doping changes the ratio of electrons to holes — it does not add net charge, and the product nₑ×nₕ stays fixed for a given temperature.

P-N Junction and Diode

Depletion region

At a p-n junction, electrons and holes diffuse across and recombine, leaving a thin layer (≈1 μm) of immobile ions with no free carriers. Its built-in barrier potential is about 0.7 V for silicon and 0.3 V for germanium.

Forward bias (p side positive): the barrier lowers and current flows once the applied voltage exceeds ~0.7 V (Si).
Reverse bias (n side positive): the barrier thickens and only a tiny leakage current flows until breakdown.
Diode current:
$I = I_0\left(e^{eV/kT} - 1\right)$
.
An ammeter in forward bias shows milliamps; reverse bias shows microamps or less.

I = I_0\left(e^{eV/kT} - 1\right)

Diode equation

Rectifiers, LED, Photodiode and Solar Cell

Rectification

Converting AC to DC. A half-wave rectifier conducts for one half-cycle only; a full-wave rectifier (centre-tap or bridge) conducts on both halves, giving a smoother DC output.

LED: emits light when forward biased — energy released as a photon, so the material must have a direct band gap (e.g. GaAs, GaN).
Photodiode: operated in reverse bias; incident light increases the reverse current.
Solar cell: a p-n junction that converts light to electricity, operating in the photovoltaic (fourth-quadrant) region with no external bias.

Which bias?

LED works forward-biased, photodiode reverse-biased, solar cell works as a generator. This 'who is biased which way' distinction is a recurring board and JEE favourite.

Transistors and Amplification

Transistor

A three-layer device in n-p-n or p-n-p form with three terminals: emitter (E), base (B) and collector (C). A small base current $I_B$ controls a much larger collector current $I_C$ , giving current gain $\beta = I_C/I_B$ .

In the common-emitter (CE) configuration the emitter is common to input and output.
Voltage gain:
$A_v = -\beta\,\frac{R_C}{R_{\text{in}}}$
— the minus sign means the output is 180° out of phase with the input.
The base region is thin and lightly doped; the emitter is heavily doped.

I_E = I_B + I_C,\quad \beta = \frac{I_C}{I_B},\quad A_v = -\beta\,\frac{R_C}{R_{\text{in}}}

Transistor currents and CE voltage gain

Logic Gates

Logic gates

Digital circuits that work with two voltage levels, 0 (low) and 1 (high). The basic gates are NOT, AND, OR, NAND, NOR and XOR, each with a truth table defining its output.

AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts.
NAND = AND followed by NOT; NOR = OR followed by NOT.
NAND and NOR are universal gates — every other gate can be built from either one alone.
XOR outputs 1 when the inputs differ.

Universal-gate question

A standard exam question: 'Which single gate can build everything else?' The answer is NAND or NOR. Remember NAND = AND + NOT.

Solved Examples

Example: In a common-emitter amplifier, $\beta = 100$ , $R_C = 2\,\text{k}\Omega$ and $R_{\text{in}} = 1\,\text{k}\Omega$ . Find the voltage gain.

Solution: $A_v = -\beta\,\frac{R_C}{R_{\text{in}}} = -100 \times \frac{2000}{1000} = -200$ . The magnitude is 200 and the negative sign shows the output is an inverted (180° phase-shifted) copy of the input.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

Diode current

I = I_0\left(e^{eV/kT} - 1\right)

Transistor currents

I_E = I_B + I_C

Current gain

\beta = \frac{I_C}{I_B}

CE voltage gain

A_v = -\beta\frac{R_C}{R_{\text{in}}}

Output waveform (full-wave)

f_{\text{ripple}} = 2f_{\text{input}}

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

Band gaps: Si ≈ 1.1 eV, Ge ≈ 0.7 eV.
Forward-bias barrier: ~0.7 V (Si) and ~0.3 V (Ge).
Photodiode: reverse bias; LED: forward bias; solar cell: no external bias.
β = I_C/I_B; CE amplifier output is inverted (phase shift π).
NAND and NOR are universal gates.
At higher temperature, semiconductor conductivity increases.

FAQ

Common questions

What is the depletion region?

It is the thin ionised layer at a p-n junction with no free charge carriers. It contains immobile positive and negative ions and creates the built-in barrier potential.

Why does a p-n junction conduct in only one direction?

Forward bias shrinks the depletion region and barrier so carriers can cross; reverse bias widens it, blocking current. This asymmetry is the basis of rectification.

What is the difference between a Zener diode and an ordinary diode?

An ordinary diode is used forward-biased; a Zener is specially doped to work in the reverse-breakdown region, where the voltage stays almost constant — used for voltage regulation.

Why are NAND and NOR called universal gates?

Because any digital function — NOT, AND, OR, XOR — can be realised using only NAND gates (or only NOR gates), so whole circuits can be built from them alone.

Continue learning

Test yourself

MCQ mock test

Exam-style questions for this chapter — no login required. Submit to see your score instantly.

Chapter mock test

Check how much of this chapter you have actually locked in — exam-style questions with instant scoring.

15 questions (of 24)~23 minNo login needed

Mastering this chapter with live help

Notes help, but doubts clear fastest in a live class. Narayan Gurukul Academy (ClassApna) runs small-batch CBSE, JEE and NEET coaching from our Mohali centre and online — with daily doubt support and mock tests.

Book a Free Demo See plans & pricing

One-on-one guidance available · Live online classes across India