Describe the two main types of semiconductors and contrast their conduction mechanism .

Explanation

The solids which are having moderate conductivity between insulators and conductors are called semi conductors .
`to ` These have the conductivity range from `10^(-6)` to `10^(4) Ohm^(-1) m^(-1)` .
`to` By doping process the conductivity of semi conductors increases E.g. : Si , Ge , crystal .
Semi conductors are of two types . They are :
1 . Intrinsic semi-conductors : In case of semi-conductors , the gap between the valence band and conduction band is small . Therefore , some electrons may jump to conduction band and show some conductivity . Electrical conductivity of semi-conductors increases with rise in temperature , since more electrons can jump to the conduction band . Substances like silicon and germanium show this type of behaviour and are called intrinsic semi-conductors .
2 . Extrinsic semi-conductors : Their conductivity is due to the presence of impurities .
They are formed by "doping" .
Doping : Conductivity of semi-conductor is too low to be of pratical use . Their conductivity is increased by adding an appropriate amount of suitable impurity . This process is called "doping".
Doping can be done with an impurity which is electron rich or electron deficient .
Extrinsic semi-conductors are of two types .
a) n-type semi-conductors : It is obtained by adding trace amount V group element (P, As , Sb) to pure Si or Ge by doping .
When P , As , Sb (or ) Bi is added to Si or Ge some of the Si or Ge in the crystal are replaced by P or As atoms and four out of five electrons of P or As atom will be used for bonding with Si or Ge atoms while the fifth electron serve to conduct electricity .
b) p-type semi-conductors : It is obtained by doping with impurity atoms containing less electrons i.e., III group elements (B , Al , Ga or In) .
When B or Al is added to pure Si or Ge some of the Si or Ge in the crystal are replaced by B or Al atoms and four out of three electrons of B or Al atom will be used for bonding with "Si" or Ge atoms while the fourth valence electron in missing is called electron hole (or) electron vacancy . This vacancy on an atom in the structure migrates from one atom to another . Hence it facilitates the electrical conductivity .