Sunday, May 20, 2012

n-Type Semiconductor

       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.
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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