When a small amount of pentavalent impurity is added to a pure
semiconductor, it is called n-type semiconductor. The pentavalent
impurity has five valence electrons. These elements are such as arsenic,
bismuth, phosphorous and antimony. Such an impurity is called donor
impurity.
Consider
the formation of n-type material by adding arsenic (As) into silicon
(Si). The arsenic atom has five valence electrons. An arsenic atom fits
in the silicon crystal in such a way that its four valence electrons
from covalent bonds with four adjacent silicon atoms. The fifth electron
has no chance of forming a covalent bond. This space electron enters
the conduction band as a free electron. Such n-type material formation
is represented in the Fig. 1. This means that each arsenic atom added
into silicon atom gives one free electron. The number of such free
electrons can be controlled by the amount of impurity added to the
silicon. Since the free electrons have negative charges, the material is
known as n-type material and an impurity donates a free electron hence
called donor impurity.
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| Fig.1 n-type material formation |
1.1 Conduction in n-Type Semiconductor
When the voltage is applied to the n-type semiconductor, the free
electrons which are readily available due to added impurity, move in a
direction of positive terminal of voltage applied. This constitutes a
current. Thus the conduction is predominantly by free electrons. The
holes are less in number hence electron current is dominant over the
hole current. Hence in n-type semiconductors free electrons are called
majority carriers while the holes which are small in number are called
minority carriers. The conduction in n-type material is shown in the
Fig. 2.
 |
| Fig. 2 Conduction in n-type material |
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