What is a semiconductor ? Describe the two main types of semiconductors and contrast their conduction mechanism.

Explanation

Semiconductors: A semiconductor is a solid that has a conductivity intermediate between that of a conductor and an insulator. The conductivity varies from `10^(-6)` to `10^(4) (Omegam)^(-1)`. In other words, due to absence of free electrons, solid substance behave as insulator, but at higher temperature it can conduct electric current, because some covalent bonds break.

Types of Semiconductors:
i. n-type semiconductors : It is obtained by doping silicon with a group 15 element. Suppose silicon is doped with phosphorus which has 5 valence electrons. Out of 5 valence electrons, only 4 valence electrons are involved in bond formation, fifth electron is not bond anywhere and can be easily promoted to the conduction band. The conduction is, thus, mainly caused by the movement of electrons. So due to presence of the negative charge such type of semiconductor is called n-type semiconductor.

p-type semiconductors : It is obtained by doping silicon with a group 13 element. Suppose silicon is doped with gallium which has three valence electrons. Three valence electrons form three bonds with neighbouring Si atoms. This gives rise to electron deficient site or positive hole. The movement of the one electron into the vacancy leaves behind a hole which carries a positive charge. The movement of positively charged holes is responsible for the conduction of charge. So, such type of semiconductor is called p-type semiconductor.