‘P’ AND ‘N’ TYPE MATERIALS

P-type and n-type germanium or silicon is used in the construction of transistors. Now question arise that what is transistors?  For that we have to first understand concept of P-type and N-type materials. These both types of materials are made by adding impurities of certain elements to the pure germanium and silicon. The act of adding an element as an impurity to the germanium or silicon is called doping.

(a)    P- Type Material :

When a trivalent element such as indium or gallium is added to tetravalent germanium or silicon as an impurity then a deficit of an electron is produced for each indium atom. The deficit of electron is termed as ‘hole’. The ‘holes’ producing impurity atoms are called acceptor atoms and the material consisting of acceptor atoms is called p-type material.

(b)    N- Type Material :

When a pentavalent element such as arsenic or antimony is added to tetravalent germanium or silicon as an impurity then a surplus electron is produced by each arsenic atom. The fifth electron of the impurity atom does not find any place in the covalent structure of the semi-conductor and it starts to wander as a free electron in the crystal. ‘Free electrons’ producing impurity atoms are called donor atoms and the material consisting of donor atoms is called N-type, material.

P and N type material are also known as extrinsic materials.

Semi - conductor

Materials having a conductivity of the order of somewhere between that of conductors and insulators are called semi-conductors. Principal semi-conductor elements are germanium and silicon. These two elements are essentially required for the manufacture of transistors.

Characteristics of semi-conductors
(a)    Semi- conductor s has a crystalline structure.
(b)    The conductivity of semi-conductors varies in accordance to temperature variations.
(c)    The conductivity of semi-conductors is greatly affected by light rays of high intensity, ultra- violet rays and infra-red rays.

Germanium and silicon 

The atomic number of germanium is 32; it means it had 32 arbitrary electrons. The order of distribution of electrons in various orbits or germanium is 2, 8, 18, and 4. Similarly the atomic number of silicon is 14 and its order of distribution of electrons in various orbits is 2, 8, and 4. In this way it is evident that both the elements have equal number of valence electrons which is 4. It means that both the elements are tetravalent The electrons of germanium and silicon atoms are bonded in covalent bonds and the number of free electrons is almost nil. Consequently, pure silicon is almost insulators, but their conductivity can be increased by adding impurities of other elements to them. Pure semi-conductors are called intrinsic.