A very poor conductor of electricity is called an insulators; an excellent conductor is a metal; and a substance whose conductivity lies between these extremes is a semiconductor. A material may be placed in one of these three classes, depending upon its energy-band structure.
Insulators
The energy band structure of insulator is indicated in schematically in Fig.1 (left side).
For a diamond crystal the region containing no quantum states is several electron volts high(6ev).
This large forbidden band separates the filled valence region from the vacant conduction band.
The energy which can be supplied to an electron from an applied field is too small to carry the particle from the filled into the vacant band.
Since the electron can't acquire sufficient energy, conduction is impossible, and hence diamond is an insulator.
Semiconductor
A substance for which the width of the forbidden energy region is relatively small(1ev) is called semiconductor.
Graphite, a crystalline form of carbon but having a crystal symmetry which is different from diamond, has such a small value of Eg, and it is a semiconductor.
As the temperature is increased, some of these valence electrons acquire thermal energy greater than Eg, and hence move into the conduction band.
These are now free electrons in the sense that they can move about under the influence of even a small applied field.
These free or conduction electrons are indicated in Fig.1(Middle) and the insulator has now become slightly conducting; it is semiconductor.
Metal
A solid which contains a partly filled band structure is called a metal. Under the influence of an applied electric field the electrons may acquire additional energy and move into higher states.
Since these mobile electrons constitute a current, this substance is a conductor and the partly filled region is the conduction band.
One example of the band structure of a metal is given in Fig.1(right side) which shows overlapping valence and conduction bands.
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