Band theory of conduction

Electrons orbit the positive nucleus of an individual atom in permitted energy levels, as shown by the grey lines on the left of the diagram below.

In a large collection of atoms, eg a metal wire or a semiconductor crystal, the energy levels become reorganised into two bands. This is shown on the right of the diagram:

  • the valence band is the lower energy levels of electrons
  • the conduction band is the higher energy levels of electrons
On the left hand side are horizontal lines that space closer together as the energy levels increase.

Electrons can’t exist in the energy 'gap' between bands.

Conduction is a movement of electrons in a solid. For conduction to occur there must be:

  • electrons free to move in the conduction band
  • spaces in energy bands for electrons to move into