Semiconductor Electronics

Lesson

Part 1 — why some solids conduct, some do not, and some can be switched: bands, doping and the p-n junction.

In this lesson you will

Explain how energy bands arise and classify a solid as a conductor, insulator or semiconductor from its band gap
Describe intrinsic and extrinsic semiconductors, holes, doping and the majority and minority carriers of n-type and p-type material
Explain the p-n junction, forward and reverse bias and the V–I characteristics, and solve simple diode-circuit numericals

Concept

Energy bands from atomic levels

An isolated atom has sharp energy levels. In a crystal, the electrons of neighbouring atoms interact, and the exclusion principle does not let N electrons share one state, so every level splits into N closely spaced levels. For a large crystal these merge into a band.

The band that holds the outer (valence) electrons is the valence band; the next band up is the conduction band.
Between them lies the forbidden gap Eg, where there are no electron states.
ISC treats energy bands qualitatively: you need the picture, not the theory.

Table of how atomic energy levels spread into bands as atoms form a crystal
Stage	What happens to the energy levels
One isolated atom	Sharp, separate energy levels
N atoms brought together	Electrons of neighbouring atoms interact; each level splits into N close levels
N very large (a crystal)	The N levels merge into a band; bands are separated by forbidden gaps
Outer electrons	Fill the valence band; the next band up is the conduction band

Diagram

Conductors, insulators and semiconductors

The size of the forbidden gap decides how a solid behaves.

Conductor: bands overlap, so electrons are free at all temperatures.
Insulator: a full valence band, an empty conduction band and a large gap (diamond: 5.5 eV).
Semiconductor: the same, but with a small gap: germanium 0.7 eV, silicon 1.1 eV. At 0 K it is an insulator.

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