Impurity types

The nature of semiconductor material can be adjusted by adding small amounts of impurities of an appropriate type, thus increasing the conductivity of the material. Two types of impurity used are n-type and p-type:

n-type semiconductor

An impurity atom of valency five (e.g. P) takes the place of a normal silicon atom in a silicon lattice. One electron is left over when all the covalent bonds have formed with neighbouring silicon atoms. These donor impurities contribute loosely bound donor electrons, which have an energy very near the top of the forbidden gap. These donor electrons are easily promoted by thermal excitation to the conduction band, leaving holes behind.

p-type semiconductor

An impurity atom of valency three (e.g. B) takes the place of a normal silicon atom in a silicon lattice. One covalent bond is missing when all the covalent bonds have formed with neighbouring silicon atoms, this represents a vacancy or a hole. These acceptor impurities create acceptor energy levels very near the bottom of the forbidden gap. These acceptor levels are easily filled by electrons thermally excited from the valence band, leaving holes behind in the valence band.