1.9.4 Semiconductors : Electrical conductivity
of a semiconductor material is intermediate
between that of metals and insulators. The
metalloids Si and Ge are semiconductors.
Like insulators, the valence band in
semiconductor is completely filled with
electrons and conduction band is empty.
However, the energy gap between the two
bands is smaller than that in an insulator. (Fig.
1.25)
the valence band than at lower temperature.
In fact semiconductors are insulators at
low temperatures and conductors at high
temperatures.Remember...
Electrical conductivity of
metals decreases and that of
semiconductor increases with increasing
temperature.1.9.5 Extrinsic semiconductors and doping :
The conductivity of a semiconductor
can be increased by doping. The process of
addition of minute quantity of impurities to a
semiconductor to increase its conductivity is
called doping. The added impurity is called
dopant.
A doped semiconductor, having higher
conductivity than pure intrinsic semiconductor,
is an extrinsic semiconductor.
There are two types of extrinsic
semiconductors, namely, n-type and p-type
semiconductors.
i. n-type semiconductor : n-type
semiconductor contains increased number of
electrons in the conduction bond.
An n-type semiconductor is obtained
by adding group 15 element to intrinsic
semiconductor which belongs to group 14.Fig. 1.25 : Semiconductors
At a temperature above absolute zero a
few electrons in the valence band have enough
thermal energy to jump through the small band
gap and occupy higher energy conduction
band. The conduction band, thus, becomes
partially filled and the valence band becomes
partially empty.
The electrons in conduction band are free
to move. When electric potential is applied to
a semiconductor, it conducts a small amount
of electicity.
Such a pure semiconductor material which
has a very low but finite electrical conductivity
is called intrinsic semiconductor.
The electrical conductivity of a
semiconductor increases with increasing
temperature. This is because, the number
of electrons with sufficient energy so as
to get promoted to the conduction band
increases as temperature rises. Thus, at higher
temperatures, there are more mobile electrons
in the conduction band and more vacancies in
Can you tell?
Let a small quantity of
phosphorus be doped into pure
silicon.- Will the resulting material contain
the same number of total number of
electrons as the original pure silicon? - Will the material be electrically
neutral or charged?
Consider, for example, doping of Si
with phosphorus. Si has a crystal structure in
which each Si atom is linked tetrahedrally to
four other Si atoms. When small quantity of