N AND P TYPE SEMICONDUCTORS

N-TYPE SEMICONDUCTORS: -

N-type semiconductors is prepared by adding pentavalent impurities like Antimony or Arsenic in pure semiconductor. P-type semiconductor is prepared by adding trivalent impurities like Aluminium, Gallium or Indium.
The figure shows N-type semiconductor in which two Arsenic atoms have replaced two Germanium atoms in the lattice structure of Ge crystal. Four of the five valence electrons of As atom are used up in forming covalent bonds and the fifth electron can act as a free electron with 0.01 eV energy. This energy is 0.05 eV in case of Silicon atom. This much energy is easily available at room temperature as thermal energy.

The pentavalent impurity is called donor impurity as it denotes the electric charge carrier electron, to the host atom. It is added in proportion of 1 in 106 pure atoms. Hence in one mole of crystal, about 1017 impurity atoms and 1017 free electrons are present. A good conductor like copper contains nearly 1023 free electrons per mole. Besides these, some more free electrons and equal number of holes result from breaking of covalent bonds. As their number is very small as compared to the free electrons from the impurity atoms, electrons are the majority charge carriers and holes minority charge carriers in the case of N-type semiconductors ( n e > n h ).

P-TYPE SEMICONDUCTORS: -

If trivalent impurities like Aluminum is
added to Ge or Si, then three free electrons of this impurity atom form covalent bonds with its neighbouring three Ge or Si atoms. Thus there is a deficiency of one electron in the formation of the fourth covalent bond. This deficiency of electron can
considered as a hole which is present in one of the bonds between the aluminium and Ge or Si atoms. This hole has a tendency to attract electron. Hence aluminium atom is known as acceptor impurity. Here, holes which behave as positively charged particles are majority charge carriers and electrons are minority charge carriers. Hence such a semiconductor is known as P-type semiconductors  (n e > n h). The figure shows symbolic representation of aluminium impurity added to Ge crystal lattice.