classification of solids according to band theory

Classification of Solids According to BAND Theory

For the conduction of electricity through a substance, we need a large
or of free electrons in it. The electrons which are closer to the nucleus of
An atom are more tightly bound than those which are further Thus the
electrons lying closer to the nucleus are not likely to take part in conduction
Since the valence electrons lie still enough close to the nucleus, they do not
e part in conducting electricity. But the conduction electrons, which lie fr
m the nucleus, predominantly take part in the conduction. Due to these
m the width of various energy band in solids determines the electrical
ductility of the solid. On the basis of the widths of the energy bands in
solids, the solids are classified into the following three categories in terms of their electrical conductivity.
a) Conductor: The conductors are
those solids in which the conduction   
band and valance band overlap each
other as in figure o Energy of
forbidden gap (E) is zero Since
there 0 physical distinction
between the conduction band and the
valence band, the overlapping results
an availability of a large number of
conduction electrons. As a result
even a slight potential difference across the substance causes the free
electrons to move, thus causing an electric current. This implies that
the valence electrons in conductors act as conduction electrons
Examples of good conductors are Cu, Al, brass etc. Usually all metals are good conductors of electricity
All metals having one or three electrons in their valence orbit have their
valance band half filled and the conduction band half filled. Therefore,
all these metals act as good conductor. For the bivalent metals like Mg
and Be with overlapping conduction and valence bands, the conductivity
is slightly less than those of monovalent and trivalent atoms.

b) Insulator: The solid substance in which
there is a wide forbidden gap between the
conduction band and the valence band as
shown in the figure no. 5 is called insulator
Energy of forbidden gap (E) is greater
than eV In the insulator, the valance
electron are bounded more tightly with
the nucleus The conduction band is empty
and the valence band is completely filled.
An increase in temperature causes few
electrons to jump from the valance band to the conduction band Hece
increase in temperature increased conductivity or decreases resistivity
This accounts for the negative temperature coefficient of resistance for
Uw insulators
At mal temperature, no conduction electrons are available To make it
conducting a very high electric potential is to be applied to take some of the
valance band electrons to conduction band Thus the insulators have very
wph resistance Examples of insulators are wood, sand, glass etc

(C) Semiconductor
semiconductorsubstance is one whose electrical properties
lie in between those of insulator and good
conductor In terms of energy band, the
forbidden band (FB) is very small as
compared to that of an insulator see figure
No. 6 Energy of forbidden gap (E) is in
order of 1 eV. However, the conduction
band is empty and the valance band is
completely filled at absolute zero of
Temperature Hence, at absolute zero, a semiconductor acts as an
insulator When temperature increases, some of the valence electrons
are able to c over the small forbidden band gap and reach the
conduction band Hence, increase in temperature increases the
conductivity or decreases the resistivity This also explains w
semiconductor has negative temperature coefficient of resistance
dhe forbidden energy band gap is small, comparatively less electric field
is required to lift the valence electron to conduction band in order for
current conduction. Therefore, the conductivity (or resistivity) of a
conductor
Semiconductor lies in between those of the insulator and the good Conductor.
Si and Ge are some examples.