Explain the formation of energy band diagram in case of conductor and semiconductor.

Explanation

Conductor :
`(a)` The energy band diagram for a metal may be in one of two forms. The valence band may be partially full, as in Fig. `(a)` or the conduction and valence bands overlap `(E_(G)=0)`, as in Fig. `(b)` . In both cases, there are a large number of electrons as well as a large number of empty energy states which these electrons can move into. For bidden energy gap is zero.
`(b)` When an electric field is applied to the material, large number of electrons are available for conduction state.
`(a)`
`(c )` Since a continuous range of unfilled energy states in available to the valence electrons, these can absorb even small energy increments and be available for conduction.
`(d)` Such a material is , therefore, a good conductor of electricity. In comparison to an insulator, the number of conduction electrons per unit volume and the conductivity in metals are very large, about `10^(28)` per `m^(3)` and greater than `10^(4)S//m`, respectively.
Semiconductor :
`(a)` At or near the absolute zero of temperature, the valence band of a semiconductor is fully occupied while the conduction band it totally empty.
`(b)` The two bands are separated by a forbidden energy gap `(E_(G))` of the order of `1 eV`.

`(c )` However, even an electron occupying a state near the top of the valence band cannot acquire energies of this mangnitude from an applied electric field.
`(d)` Hence, the valence band remains full and the conduction band empty. With no free electrons in the conduction band, conduction is impossible and the material behaves as a perfect insulators at or near `0 K`.
`(e)` At room temperature few covalent bonds are broken because of thermal energy and valence electrons are raised to conduction band.
`(f)` When potential difference is applied the electrons and holes behaves as charge carriers, but current flowing is very small.