n type semiconductor definition

n-type Semiconductor

When a small amount of pentavalent impurity is added to a pure semiconductor, it is
known as n-type semiconductor.
The addition of pentavalent impurity provides a large number of free electrons in
the semiconductor crystal. Typical examples of pentavalent impurities are arsenic (At.
No. 33) and antimony (At. No. 51).

Such impurities which produce n-type semiconductor
are known as donor impurities because they donate or provide free electrons to the
semiconductor crystal.

To explain the formation of n-type semiconductor, consider a pure germanium
crystal. We know that germanium atom has four valence electrons. When a small
amount of pentavalent impurity like arsenic is added to germanium crystal, a large
number of free electrons become available in the crystal. The reason is germanium
crystal in such a way that its four valence electrons form covalent bonds with four
germanium atoms. The fifth valence electron of arsenic atom finds no place in co-valent
bonds and is thus free as shown in Fig.1. Therefore, for each arsenic atom added,
one free electron will be available in the germanium crystal. Though each arsenic atom
provides one free electron, yet an extremely small amount of arsenic impurity provides
enough atoms to supply millions of free electrons.

n type semiconductor definition
Fig-2 shows the energy band description of n-type semi-conductor. The
addition of pentavalent impurity has produced a number of conduction band electrons
i.e., free electrons. The four valence electrons of pentavalent atom form covalent bonds
with four neighboring germanium atoms. The fifth left over valence electron of the
pentavalent atom cannot be accommodated in the valence band and travels to the
conduction band.

The following points may be noted carefully:

(i) many new free electrons are produced by the addition of pentavalent
impurity.
(ii) Thermal energy of room temperature still generates a few hole-electron
pairs. However, the number of free electrons provided by the pentavalent
impurity far exceeds the number of holes. It is due to this predominance of
electrons over holes that it is called n-type semiconductor (n stands for
negative).

n-type conductivity

The current conduction in an n-type semiconductor is
predominantly by free electrons i.e. negative charges and is call n-type or electron
type conductivity. To understand n-type conductivity, refer to. When p.d. is apply across the n-type semiconductor, the free electrons donated by impurity) in the
crystal will be direct towards the positive terminal, constituting electric current. As the
current flow through the crystal is by free electrons which are carriers of negative
charge, therefore, this type of conductivity. It may be note that conduction is just as in
ordinary metals like copper.

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