Semiconductors are a unique class of materials in which the number of charge carriers, the particles that move electric charge, is intermediate between those of insulators and those of conductors. A conductor is a material that contains a large number of free electrons that are able to move freely when a voltage is applied. An insulator is a material that has very few free electrons and hence is a poor conductor of electricity. There are two main types of semiconductors: intrinsic and extrinsic semiconductors. The conducting power of intrinsic semiconductors is greatly influenced by temperature and sometimes light making them particularly interesting.

Although there are some materials that are semiconductors in their pure state, they are virtually never used as such in electronic devices because the only charge carriers present are those produced by thermal breakdown of the covalent bonds within the material. Two of the most common...