Doping

Doping is the practice of adding impurities (dopants) to semiconductors to control their electrical conductivity. If we add extra electrons, we call them n-type semiconductors and if we remove electrons (add extra holes), we call them p-type semiconductors (both for negative and positive).

N-type

Remember that silicon has 4 valence electrons, To create n-type compounds, we add phosphorous, since it carries extra electrons (to the right on the periodic table).¹ The extra electron is weakly bound and can be easily promoted. For this reason, we say that it has an extra donor energy level within the band gap. N-types have a conductivity given by:

$${\sigma }_n=nq{\mu }_n$$

P-types

We dope with boron to create extra holes. This means we can promote electrons into the extra hole for every dopant atom. Just as n-types have a donor level, p-types have an acceptor level in their band gap near the valence band. And its conductivity is given by:

$${\sigma }_p=pq{\mu }_p$$

Footnotes

1. From Engineering Chemistry and Materials Science, by Scott Ramsay. ↩