JEE Main & Advanced Physics Semiconducting Devices Extrinsic Semiconductor

(1) An impure semiconductor is called extrinsic semiconductor

(2) When pure semiconductor material is mixed with small amounts of certain specific impurities with valency different from that of the parent material, the number of mobile electrons/holes  drastically changes. The process of addition of impurity is called doping.

(3) Pentavalent impurities : The elements whose atom has five valance electrons are called pentavalent impurities e.g. As, P, Sb etc. These impurities are also called donor impurities because they donate extra free electron.

(4) Trivalent impurities : The elements whose each atom has three valance electrons are called trivalent impurities e.g. In, Ga, Al, B, etc. These impurities are also called acceptor impurities as they accept electron.

(5) The compounds of trivalent and pentavalent elements also behaves like semiconductors e.g. GaAs, InSb, In P, GaP etc.

(6) The number of atoms of impurity element is about 1 in \[{{10}^{8}}\] atoms of the semiconductor.

(7) In extrinsic semiconductors  \[{{n}_{e}}\ne {{n}_{h}}\]

(8) In extrinsic semiconductors fermi level shifts towards valence or conduction energy bands.

(9) Their conductivity is high and they are used for practical purposes.

(10) In a doped extrinsic semiconductor, the number density of \[{{e}^{-}}\]of the conduction band \[({{n}_{e}})\] and the number density of holes in the valence band \[({{n}_{h}})\] differs from that in a pure semiconductor. If \[{{n}_{i}}\] is the number density of electron in conduction band or the number density of holes in valence band in a pure semiconductor then \[{{n}_{e}}{{n}_{h}}=n_{i}^{2}\](mass action law)

(11) Extrinsic semiconductors are of two types

(i)  N-type semiconductor  

(ii) P-type semiconductor