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Conductors

In a conductor there are no band gaps between the valence and conduction bands. In some metals the conduction and valence bands partially overlap. This means that electrons can move freely between the valence band and the conduction band.

The conduction band is only partially filled. This means there are spaces for electrons to move into. When electrons for the valence band move into the conduction band they are free to move. This allows conduction.

Insulators

An insulator has a large gap between the valence band and the conduction band.

The valence band is full as no electrons can move up to the conduction band. As a result, the conduction band is empty.

Only the electrons in a conduction band can move easily, so because there aren't any electrons in an insulator's conduction band, the material can't conduct.

Semiconductors

In a semiconductor, the gap between the valence band and conduction band is smaller. At room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band. This allows some conduction to take place.

An increase in temperature increases the conductivity of a semiconductor because more electrons will have enough energy to move into the conduction band.

The difference between insulators and semiconductors is due to a small amount of impurity added to a semiconductor which affects the energy bands. This process is called doping.