Extrinsic Semiconductor

To convert this stable crystal into a semiconductor some impurity is added into it by doping the silicon material. For adding the impurity, both silicon and impurity are melted at high temperature and mixed perfectly. Then the mixture is cooled to form a new type of material known as the semiconductor. It is then cut into the shape of thin semiconductor wafers. It is also called as extrinsic semiconductor, because some external material is added into pure Silicon and a pure silicon crystal is called as the intrinsic semiconductor.

The impurity added into pure silicon is mainly of two types: the trivalent and the pentavalent. In trivalent impurity likeAluminium, Gallium, Indium and Boron, the outermost shell can accept one electron, to become stable. It means that such atom is accepter type atom. Similarly, in pentavalent impurity like Bismuth, Arsenic, Antimony and Phosphorus the outermost shell can give out one electron to become stable. It means that such atom is donor typeatom.

To manufacture N–type semiconductor, pentavalent impurity is added into a pure Si crystal. Due to this, four electrons form covalent bond with four neighboring silicon atoms and one electron becomes a free electron within four Silicon atoms. This free electron is far away from the nucleus of silicon atom and it is free from the covalent bond. Thus, it can carry current through N-type semiconductor, when voltage is applied to it. Hence, in N–type semiconductor, majority charges are electrons to carry current and minority charges are the holes.

In the same way, P–type semiconductor is manufactured. For this, trivalent impurity (like Aluminium) is added into a pure Si crystal. Thus, three valance electrons of Aluminium, form covalent bond with three neighboring Silicon atoms but now one electron becomes LESS for Aluminium, within four Silicon atoms. This vacancy produced due to absence of one electron is called as hole. This hole is considered as +ve charge, because it can accept one electron from outside. Thus, it can carry current through P-type semiconductor, when voltage is applied to it. Hence, in P–type semiconductor, majority charges are holes to carry current and minority charges are the electrons.

For more details: Extrinsic Semiconductor