Semiconductor ElectronicsNEET Physics · Class 12 · NCERT Chapter 14

Introduction

Almost every electronic device you own (phone, laptop, TV remote, calculator, charger) runs on semiconductors. This chapter covers the basics: how energy bands explain conductors / insulators / semiconductors, how doping turns pure Si into n-type or p-type, what happens at a p-n junction, how diodes rectify AC, and how logic gates perform digital operations.

Expect 1 to 2 NEET questions every year. Common asks: identifying n-type vs p-type, knee voltage of Si and Ge, output frequency of half-wave vs full-wave rectifiers, Zener diode use, and logic gate truth tables.

Energy bands in solids

Many atoms close together share their valence electrons, and discrete atomic energy levels broaden into bands. The band where electrons fill at the highest energy is the valence band; the next empty band above is the conduction band. The gap between is the band gap E_g.

• Conductors: bands overlap (E_g = 0). Many free electrons. e.g. Cu, Al.
• Semiconductors: small gap, ~1 eV. Si: 1.1 eV. Ge: 0.7 eV.
• Insulators: large gap, > 3 eV. Diamond: 5.5 eV.

Solids have allowed energy bands separated by forbidden gaps. The size of the gap between the topmost filled valence band and the empty conduction band tells you whether the material conducts.

Intrinsic semiconductors

Pure Si or Ge has equal numbers of electrons and holes:

n_i (intrinsic carrier concentration) depends on T and E_g. At 300 K: Si ~10¹⁰ /cm³, Ge ~10¹³ /cm³. So Ge has more carriers but is also less stable (smaller gap means more thermal noise).

Extrinsic semiconductors (doping)

Add a tiny fraction of a different-valence atom (one in 10⁶ Si atoms is a typical ratio). Two cases:

• n-type: pentavalent dopant (P, As, Sb, Bi). The 5th electron is loosely bound and free at room T. Majority: electrons. Minority: holes.
• p-type: trivalent dopant (Al, B, Ga, In). One bond has a missing electron, called a hole. Majority: holes. Minority: electrons.

Pure Si is a poor conductor. Adding pentavalent dopant gives n-type (extra electrons); trivalent gives p-type (extra holes).

Mass-action law

At equilibrium, the product is fixed by temperature alone. So in n-type, n_h = n_i² / N_D drops dramatically. This is why minority carriers are so few.

p-n junction formation

Bring p- and n-type material into intimate contact. Electrons diffuse from n to p; holes from p to n. They recombine near the junction, leaving immobile dopant ions. This region (~ 0.1 µm) is the depletion region. The ions create a built-in barrier potential that stops further diffusion at equilibrium.

• Si: barrier ≈ 0.7 V
• Ge: barrier ≈ 0.3 V

Forward and reverse biasing

• Forward bias: + on p, − on n. Reduces barrier; majority carriers flow easily once V > knee voltage. Depletion narrows.
• Reverse bias: + on n, − on p. Adds to barrier. Only minority carriers cross, giving a tiny saturation current. Depletion widens. Above breakdown voltage, large current flows (Zener or avalanche).

At the p-n junction, electrons and holes recombine, leaving a thin depletion region of immobile ions. A forward bias narrows it; reverse bias widens it.

Diode I-V characteristic

For an ideal diode:

In the forward direction, current rises sharply once V exceeds the knee voltage (~0.7 V for Si). In reverse, only the saturation current I_0 flows until breakdown.

Diode I-V: forward current rises sharply after the knee voltage. Reverse current is tiny until breakdown.

Half-wave rectifier

Single diode in series with a load. Diode conducts only on the positive half of the AC cycle. Output:

Output frequency = input frequency. The output is far from steady DC; usually a capacitor filter is added.

Full-wave rectifier

Either centre-tapped (2 diodes) or bridge (4 diodes). Both halves of the AC cycle are flipped to positive.

Output frequency = 2 × input frequency. Smoother, higher average DC, twice the efficiency. Used in most power supplies.

Half-wave passes only positive halves; full-wave flips the negative half too. Output frequency is the same as input for half-wave, twice for full-wave.

Zener diode and regulator

A heavily doped p-n junction designed to operate in reverse breakdown. Beyond V_Z, voltage stays nearly constant. Use it as a regulator: input voltage with ripple goes through R_s, Zener clamps the output.

A Zener diode in reverse breakdown holds its voltage steady at V_Z. Use it as a simple voltage regulator: a series resistor drops the excess.

LED, photodiode, solar cell

Three optoelectronic devices, all p-n junctions used differently:

• LED: forward biased. Recombination releases photons of energy ~E_g.
• Photodiode: reverse biased. Light creates electron-hole pairs in depletion, increasing reverse current.
• Solar cell: no external bias. Light generates EMF; powers a load.

Three optoelectronic devices, all built around a p-n junction. Click each to see how it works.

Logic gates

Digital signals: HIGH (1) or LOW (0). The seven basic gates:

• NOT (inverter): Y = NOT A.
• AND: Y = A · B.
• OR: Y = A + B.
• NAND: Y = NOT (A · B). Universal gate.
• NOR: Y = NOT (A + B). Universal gate.
• XOR: Y = 1 when A and B differ.

Click each gate to see its symbol, rule and truth table. NAND and NOR are universal: every other gate can be built from one type.

Worked NEET problems

Summary cheat sheet

• Bands: conductor (overlap), semiconductor (~1 eV), insulator (>3 eV).
• Intrinsic: n_e = n_h = n_i.
• Mass-action: n_e n_h = n_i².
• n-type: pentavalent dopant. p-type: trivalent dopant.
• Knee voltage: Si 0.7 V, Ge 0.3 V.
• Half-wave: output f = input f. Full-wave: output f = 2 × input f.
• Avg DC half: V_m / π. Avg DC full: 2 V_m / π.
• Zener: reverse breakdown, voltage regulator.
• LED: forward bias. Photodiode: reverse bias. Solar cell: no bias.
• Universal gates: NAND and NOR.

Next: try the interactive widgets for diodes, rectifiers and logic gates, or work through the 32 NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.