Class 11 Physics — Chapter 308: Semiconductors

Key concepts: Semiconductors (Class 11 Physics)

From energy bands to working devices: how band gaps separate conductors, insulators and semiconductors, intrinsic conduction by electrons and holes, doping to make n-type and p-type material, the pn junction with its depletion region and barrier potential, forward and reverse bias, rectification, and the special-purpose diodes.

Energy bands in solids

Atomic energy levels broaden into bands when atoms come together, with a forbidden energy gap separating the filled valence band from the empty conduction band.

Conductors, insulators, semiconductors

In a conductor the bands overlap; an insulator has a gap above about 3 eV; a semiconductor's gap is small enough for thermal energy to bridge it.

Band gap

Silicon's gap is about 1.1 eV and germanium's about 0.7 eV at room temperature. The smaller gap makes germanium the more thermally sensitive of the two.

Intrinsic semiconductor

Pure semiconductor in which electrons and holes are created only in pairs by thermal excitation, so their concentrations are exactly equal.

Electrons and holes

A hole is the vacancy left by an excited electron and behaves as a mobile positive charge. Electrons have the higher mobility of the two.

Doping

Adding a controlled trace of impurity to raise the carrier concentration by orders of magnitude, which is what makes practical devices possible.

n-type semiconductor

Doped with a PENTAVALENT impurity such as phosphorus or arsenic, giving a surplus of electrons as majority carriers and holes as minority carriers.

p-type semiconductor

Doped with a TRIVALENT impurity such as boron or indium, giving holes as majority carriers. The crystal stays electrically neutral overall.

Effect of temperature

Raising the temperature frees more carriers, so a semiconductor's resistance FALLS with heating — the opposite of a metal's behaviour.

pn junction

The boundary formed within a single crystal doped p-type on one side and n-type on the other. It is the building block of every diode and transistor.

Depletion region

A thin layer at the junction swept clear of mobile carriers, containing only fixed ionised dopant atoms and therefore acting as an insulator.

Barrier potential

The built-in potential across the depletion region opposing further diffusion, about 0.7 V for silicon and 0.3 V for germanium.

Forward bias

Connecting p to the positive terminal narrows the depletion region and lowers the barrier, so appreciable current flows once the barrier voltage is exceeded.

Reverse bias

Connecting p to the negative terminal widens the depletion region and raises the barrier, so only a tiny minority-carrier leakage current flows.

Rectification

Using a diode's one-way conduction to convert AC into DC. A half-wave rectifier passes one half of each cycle; a full-wave rectifier inverts and uses both.

Special purpose diodes

The Zener diode conducts in controlled reverse breakdown and regulates voltage; the LED emits light on recombination; the photodiode and solar cell convert light into current.