Formation of Extrinsic Semiconductor Materials

Formation of Extrinsic Semiconductor Materials

It is seen that when pentavalent impurity, is added to intrinsic semiconductor, n‒type extrinsic semiconductor is obtained. While when trivalent impurity is added to intrinsic semiconductor, p‒type extrinsic semiconductor is obtained.

1. Formation of n‒type Semiconductor

• Consider a pentavalent impurity like Arsenic (As) is added to Silicon (Si). It is a donor impurity with five valence electrons.

• The arsenic atom fits in the silicon crystal in such a way that its four valence electrons form covalent bonds with four adjacent silicon atoms. Remember that both germanium and silicon have four electrons in the valence shell.

• The fifth electron of arsenic has no chance to form covalent bond hence this electron enters in conduction band as a free electron. Thus each As atom added to Si atom donates one free electron. This is shown in Fig. 1.9.1.

• The number of free electrons can be controlled by the amount of impurity added.

• Since the free electrons have negative charges, the material obtained is called n‒type extrinsic semiconductor.

2. Conduction in n‒type Material

• In intrinsic semiconductor, the number of free electrons and holes is same. But when pentavalent impurity is added in large extent, the number of free electrons becomes very high compared to the number of holes.

• Thus if voltage is applied, the current is mainly because of free electrons which are large in number.

Hence the free electrons are called majority carriers in n‒type material. While there is small current due to less number of holes. Hence the holes are called minority carriers in n‒type material.

• This is shown in Fig. 1.9.2.

3. Formation of p‒type Semiconductor

• Consider a trivalent impurity like Gallium (Ga) is (4 valence electrons) added to Silicon (Si). It is an acceptor impurity with three valence electrons.

• The gallium atom fits into the silicon crystal in such a way that its three valence electrons form covalent bonds with the three adjacent silicon atoms. There is shortage of one electron to form a covalent bond. This creates a vacancy in the fourth covalent bond which is nothing but a hole. Thus each Ga atom added into Si atom creates one hole which is ready to accept an electron. This is shown in Fig. 1.9.3.

• 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 obtained is called p‒type extrinsic semiconductor.

4. Conduction in p‒type Material

• When large trivalent impurity is added to silicon or germanium, the number of holes becomes very high as compared to free electrons.

• Thus if voltage is applied, the holes which are large in number move towards negative of battery and mainly responsible for the current. Hence the hole are called majority carriers in p‒type material while there is small current due to the movement of less number of free electrons. Hence the free electrons are called minority carriers in p‒type material.

• This is shown in Fig. 1.9.4.

Review Questions

1. Explain the formation of n‒type material and conduction in n‒type material.

2. Explain the following:

i) Formation of p‒type semiconductor.

ii) Conduction in p‒type material.