The difference in the behaviour of solids as regard their electrical conduction can be explained in terms of energy bands. The valence and conduction bands are of important bands for studying the behaviour of various solids.
Insulators :
Insulators are those materials in which valence electrons are bound very tightly to their parent atom, which does not allow the passage of electric current. In terms of the energy band, the valence band is full while the conduction band is empty. The energy gap between the valence and conduction band is very large i.e., about 6 eV. Therefore a very high electric field is required to push the valence electrons to the conduction band. For this reason , the electrical conductivity of such materials is small . The resistance of an insulator decrease with the increase in temperature i.e. , an insulator has negative temperature co efficient resistance.
2. Conductor :
Conductors are those substances which easily allow the passage of electric current through them . It is because there are a large number of free electrons available in a conductor. In terms of energy band the valence and conduction bands overlap each other . Due to this overlap slight potential difference across a conductor causes the free electrons to constitute electric current .
3. Semiconductors :
Semiconductors are those substances whose electrical conductivity lies in between conductors and insulators. In terms of energy bands, semiconductors can be defined as those materials which at room temperature have.
( a ) Partially filled conduction band
( b ) Partially filled valance band and
( c ) Very narrow energy gap of the order of leV between valence and conduction bands.
At 0 ° K there are no electrons in the conduction band and the valence band is completely filled. With the increase in temperature, some of the valence electrons acquire energy greater than Eg and cross over to the conduction band. In the absence of electrons, holes will result in a valence band. So the conductivity of the semiconductor increases with the rise in temperature ie .. a semiconductor has a negative temperature coefficient of resistance.
