When the impurities are added to the intrinsic semiconductor the
allowable energy levels are introduced and material becomes extrinsic
semiconductor.
In n-type semiconductor, a donor impurity is added. Each donor atom
donates one free electron and there are large number of free electrons,
available in the conduction band. The donor energy level corresponding
to the donor impurity added is just below the conduction band. This
donor level is indicated as ED and its distance is 0.01 eV
below the conduction band in germanium while it is 0.05 eV below the
conduction band in silicon. As this distance is very small, even at room
temperature, almost all the extra electrons from the donor impurity
atoms jump into the conduction band. Hence number of free electrons is
very large in case of n-type material. Due to abundant free electrons,
the probability of occupying the energy level by the electrons, towards
the conduction band is more. This probability is indicated by Fermi
level EF. So in n-type material, the fermi level EF gets
shifted towards the conduction band. But it is below the donor energy
level. The overall energy band diagram for n-type material is shown in
the Fig. 1(a).
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| Fig. 1 Energy band diagrams for extrinsic semicondcutor |
As
against this, in p-type material, acceptor impurity is added. Due to
this, large number of holes get created in the valence band. The
acceptor energy level corresponding to acceptor impurity gets introduced
which is indicated as EA and is very close to the valence
band just above it. At room temperature, the electrons from valence band
jump tp acceptor energy level, leaving behind the holes in valence
band. This shifts the fermi level EF towards the valence
band. It lies above the acceptor energy level. The overall energy band
diagram for p-type material is shown in the Fig. 1(b).
Note :
It must be noted that the doped material is always electrically neutral
as the total number of electrons is equal to the total number of
protons, after the addition of impurity.
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