-Insulators
-Metals
-Semiconductors
*Insulator:An insulator is a material having negligible
conductivity.Mainly the forbidden energy gap is more in insulators.If
additional energy is applied to the electron in the upper level of
valence band, then the electron crosses the forbidden energy gap and
enters the conduction band.But energy provided will be much smaller
than the forbidden energy so these are called as insulators.The number
of free electrons in an insulators is very small.
*Metals:Metal is a good conductor of electricity due to the electron
flow and metal has overlapping of valence and conduction bands.The
band occupied by the valence electrons may not be completely filled
and that there are no forbidden levels at higher energies.Depending
upon the metals, sometimes the electrons move freely in the metal.when
we apply the electrical field then these electrons get additional
energy and move to higher energy within conduction band.Hence the
additional energy required for transfer of electrons from valence band
to conduction band is extremely small , so conductivity is
excellent.Example of metal is copper metal. 18:39
