In
isolated atom the valence electrons can exist only in one of the allowed
orbitals each of a sharply defined energy called energy levels. But when two
atoms are brought nearer to each other, there are alterations in energy levels
and they spread in the form of bands.
Energy
bands are of following types
(1)
Valence band
The
energy band formed by a series of energy levels containing valence electrons is
known as valence band. At 0 K, the
electrons fills the energy levels in valence band starting from lowest one.
(i)
This band is always fulfill by electron.
(ii)
This is the band of maximum energy.
(iii)
Electrons are not capable of gaining energy from external electric field.
(iv)
No flow of current due to such electrons.
(v)
The highest energy level which can be occupied by an electron in valence band
at 0 K is called fermi level.
(2)
Conduction band
The
higher energy level band is called the conduction band.
(i)
It is also called empty band of minimum energy.
(ii)
This band is partially filled by the electrons.
(iii)
In this band the electrons can gain energy from external electric field.
(iv)
The electrons in the conduction band are called the free electrons. They are
able to move any where within the volume of the solid.
(v)
Current flows due to such electrons.
(3)
Forbidden energy gap (DEg)
Energy
gap between conduction band and valence band
(i)
No free electron present in forbidden energy gap.
(ii)
Width of forbidden energy gap upon the nature of substance.
(iii)
As temperature increases (), forbidden energy gap decreases (¯)
very slightly.
