(a) What is doping? (b) Explain how doping improves the conductivity of a semiconductor

(a) Doping is the deliberate addition of a small, controlled amount of a suitable impurity (a pentavalent or trivalent element) to a pure (intrinsic) semiconductor in order to increase the number of free charge carriers and hence its electrical conductivity.

(b) How doping improves conductivity:

A pure semiconductor such as silicon has very few free charge carriers, so it conducts poorly.

• Adding a pentavalent impurity (e.g. phosphorus, arsenic) donates extra free electrons to the crystal, producing an n-type semiconductor in which electrons are the majority carriers.
• Adding a trivalent impurity (e.g. boron, aluminium) creates vacancies called holes that act as positive carriers, producing a p-type semiconductor in which holes are the majority carriers.

In both cases the impurity greatly increases the concentration of mobile charge carriers, so more charge can flow for a given voltage and the conductivity rises sharply.