When a small amount of trivalent impurity is added to a pure
semiconductor, it is called p-type semiconductor. The trivalent impurity
has three valence electrons. These elements are such as gallium, boron
or indium. Such an impurity is called acceptor impurity.
Consider the formation of p-type material by adding gallium (Ga) into
silicon (Si). The gallium atom has three valence electrons. So gallium
atom fits in the silicon crystal in such a way that its three valence
electrons from covalent bonds with the three adjacent silicon atoms.
Being short of one electron, the fourth covalent bond in the valence
shell is incomplete. The resulting vacancy is called a hole. Such p-type
material formation is represented in the Fig. 1. This means that each
gallium atom added into silicon atom gives one hole. The number of such
holes can be controlled by the amount of impurity added to the silicon.
As the holes are treated as positively charged, the material is known as
p-type material.
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| Fig. 1 p-type material formation |
At
room temperature, the thermal energy is sufficient to extract an
electron from the neighbouring atom which fills the vacancy in the
incomplete bond around impurity atom. But this creates a vacancy in the
adjacent bond from where the electron had jumped, which is nothing but a
hole. This indicates that a hole created due to added impurity is ready
to accept an electron and hence is called acceptor impurity. Thus even
for a small amount of impurity added, large number of holes get created
in the p-type material.
1.1 Conduction in p-type Semiconductor
If now such p-type material is subjected to an electric field by
applying a voltage then the holes move in a valence band and are mainly
responsible for the conduction. So the current conduction in p-type
material is predominantly due to the holes. The free electrons are also
present in conduction band but are very less in number. Hence holes are
the majority carriers while electrons are minority carriers in p-type
material. The conduction in p-type material is shown in the Fig. 2.
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| Fig. 2 Conduction in p-type material |
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