Semi conductors are the solids that have properties intermediate between metals & insulators.They have only small difference in energy between the filled valence band & empty conduction band.The conductivity is also intermediate between that of a metal & an insulator & depends upon the number of electrons in the conduction band.

The semiconductors behaves in two types as

a) intrinsic conduction

b) extrinsic conduction

(a) Intrinsic conduction = pure silicon & germanium are poor conductors because they have network of four strong covalent bonds.But when electricity is provided to the crystals ,the electrons moves leaving behind a positive charge or hole at the site of missing electron,as a result of which the crystal will now conduct electricity

(b) Extrinsic conduction = here the conductivity of intrinsic conductors is increased by adding an appropriate amount of suitable impurity.This process is called doping.T he impurities are of 2 types:

(1) Electron rich

(2) Electron deficient

(1) Electron rich impurity – here atoms with five valence electrons are added to the semi conductor,as a result of which the silicon atoms are randomly replaced by the impurity atom.the fifth atom remains free of impurity.The extra atom of impurity has electrons which is now available for conducting electricity.This type of conductor is called n- type semi conductor.

(2) Electron deficient impurity – here an atom with three valence electrons are added to the semiconductor,as a result of which silicon or germanium atom are replaced by the impurity atom.the valence electrons of impurity will make bond with three atom but fourth atom remains free in the crystal of silicon or germanium ,which is now available for conducting electricity . This type o conductor is called p-type semi conductor.