The characteristics of semiconductor materials can be altered significantly by the addition of certain impurity atoms into relatively pure semiconductor material.
A semiconductor material that has been subjected to the doping process is called an extrinsic material.
There are two types of extrinsic materials of immeasurable importance to semiconductor device fabrication: n-type & p-type
n-type Material :
Both the n- type and p- type materials are formed by adding a predetermined number of impurity atoms into a germanium or silicon base.
The n-type is created by introducing those impurity elements that have five valence electrons (pentavalent), such as antimony, arsenic, and phosphorous.
Diffused impurities with five valence electrons are called donor atoms.
p-type Material :
The p-type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons. The elements most frequently used for the purpose are boron, gallium and indium.
The Diffused impurities with three valence electrons are called acceptor atoms.
Electron versus Hole Flow :-
If a valence electron acquires sufficient kinetic energy to break its covalent bond and fills the void created by a hole, then a vacancy, or hole, will be created in the covalent bond that released the electron. There is, therefore, a transfer of holes to the left and electrons to the right.
Majority and Minority Carriers :-
In the intrinsic state, the number of free electrons in Ge or Si is due only to those few electrons in the valence band that have acquired sufficient energy from thermal or light sources to break the covalent bond or to the few impurities that could not be removed.
In an n-type material the electron is called the majority carrier and the hole the minority carrier.
For the p-type material the number of holes far outweighs the number of electrons.
In a p-type material the hole is the majority carrier and the electron id the minority carrier.
When the fifth electrons of s a donor atom leaves the parent atom, the atom remaining acquires a net positive charge: hence the positive sign in the donor-ion representation.