Conductors

Conducting materials are those in which number of free electrons are available for electric conduction.  In terms of energy bands, it means that electrical conductors are those which have overlapping valence and conduction bands.

conductors

In fact, there is no physical distinction between the two bands. Hence, the availability of large number of conduction electrons.

A conductor has a positive temperature coefficient of resistance.

Semiconductors

A semiconductor material is one whose resistivity lie in between those of insulators and conductors. Examples are : silicon and germanium. In terms of energy bands, semiconducting materials can be defined as those materials which has almost an empty conduction band and almost filled valance band with a very narrow energy gap separating the two.

When an external field or temperature is applied to the semiconductor, width of the forbidden energy bands is decreased so that some of the electrons are liberated into the conduction band. In other words we can say that, conductivity of semiconductors increases with increases with temperature.

A Semiconductor has a negative temperature coefficient of resistance.

Insulators

Insulators are those materials in which valence electrons are bound very tightly to their parent atoms, thus requiring very large electric field to remove them from attraction of their nuclei. Insulators have an empty conduction band. The energy gap between the valence and conduction bands is very large.

insulators

An insulator has a  negative temperature coefficient of resistance.