A semiconductor is a substance with an electrical conductivity between that of a conductor and an insulator. The conductivity of a semiconductor increases as temperature increases. Adding appropriate impurities also increases conductivity.
A semiconductor consists of elements such as silicon and germanium, or compounds, such as aluminum phosphide, with a crystalline structure. At normal temperatures, some electrons break free and become carriers of the electric current. The holes (electron deficiencies) left by these electrons move in the opposite direction to the electrons and behave like positive charge carriers. Impurities are usually added to the semiconductor material in controlled amounts during manufacture to add more free electrons or create more holes. This process is called doping.
There are two types of semiconductor: the n-type, in which the current carriers (electrons) are negative; and the p-type in which the current carriers are moving, positively-charged holes. A semiconductor junction is formed when there is an abrupt change from one type of impurity to the other. Such a p-n junction acts as a very efficient rectifier and is the basis of the semiconductor diode.