What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.
Semiconductor : The solids which have intermediate conductivities between metals and non-metals i.e. between 10-6 to 104 -1 m-1 are called semiconductors.
Example : Germanium and silicon.
Main types of semiconductors are of two types :
(i) Intrinsic semiconductor : These are insulators at room temperature and become semiconductors when temperature is raised
(ii) Extrinsic semiconductor : p-type semiconductor
These are formed by doping impurity of lower or higher group.
These are subdivided into two types :
p-type semiconductors : When a silicon crystal is doped with atoms of group-13 elements like B, Al, Ga etc., the atom forms only 3 covalent bonds with the Si atom and 4th missing electron creates a hole which conducts electricity.
n-type semiconductor : When a silicon crystal is doped with atoms of group-15 elements like P, As etc., then only four of the five valence electrons of each impurity atom participate in 4 covalent bond formation and 5th e- conducts electricity.