A conductor is a material that easily conducts electrical current.Most metals are good conductors.The best conductors are single element materials,such as copper(Cu),silver(Ag),gold(Au),which are characterized by atoms with only one valence electron very loosely bound to the atom.These loosely bound valence electrons become free electrons.Therefore,in a conductive material the free electrons are valence electrons.
In terms of energy bands ,it means that electrical conductors are those which have overlapping valence and conduction bands.In fact ,there is no physical distinction between the two bands.Hence the availability of a large number of conduction electrons.
Another point worth noting is that in the absence of forbidden energy gap in good conductors,there is no structure to establish holes.The total current in such conductors is simply a flow of electrons.It is exactly for this reason that the existence of holes was not discovered until semiconductors were studied thoroughly.
What is a Semiconductor?
A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors.Example are:Germanium and silicon.
In terms of energy bands,semiconductors can be defined as those materials which at room temperature have:
- Partially filled conduction band.
- Partially filled valence band.
- A very narrow energy gap (of the order of ( 1 eV) between them.
At 0ºk ,there are no electrons in the conduction band of semiconductors and their valence band is completely filled.It means that at absolute zero temperature ,a piece of Ge or Si acts like a perfect insulator.However,with increase in temperature,width of the forbidden energy band is decreased so that some of the electrons are liberated into the conduction band.In other words,conductivity of semiconductors increases with temperature.It means that they have negative temperature coefficient of resistance.