A typical semiconductor is a crystalline solid material with an Electrical Conductivity that is highly dependent on temperature. At low absolute temperatures a pure semiconductor will appear to be a good insulator, however, its conductivity rises dramatically as the temperature increases. Semiconductors like silicon are the basis of modern electronics and integrated circuit technology.
The electrical conductivity of a solid depends on two factors. Firstly, the number of mobile charge carriers present and secondly, the mobility or speed at which the carriers move under the influence of an electric field. It is the first consideration, usually called carrier concentration, that determines whether a material is a good conductor like a metal or an insulator such as diamond.
In a typical metal each atom contributes one or more of its outer electrons to a common sea of conduction electrons that permeates the solid, yielding a very large number of conduction electrons. This sea of electrons or electron gas will readily support a transport current in the presence of an electric field. By comparison, a good insulator has strongly localized electrons. A considerable amount of energy is required to break away the electrons from their current atom sites, making transport currents through the material impossible unless work is done initially to free a significant number of electrons.
Semiconductors have resistance properties in between the above two extremes, however, they are not simply poor insulators (or bad conductors). The name is used for those materials that exhibit insulator like properties at very low temperatures, because all the outer electrons called valence electrons are localized, but begin to conduct as the temperature is raised. This is because the thermal energy is sufficient to break away electrons from their local bonds and promote them into the role of conduction electrons. Thus at higher temperatures a semiconductor exhibits properties closer to those of metals. It is the dramatic change in conductivity due to the excitation of valence electrons into the conduction state as the ambient temperature is increased that is the characteristic feature of a semiconductor. The conductivity of a pure semiconductor would, theoretically, increase almost exponentially with absolute temperature. Conduction in a semiconductor takes place both via the conduction electrons and also the valence electrons that are now able move to neighboring atoms that have lost valence electrons. The vacancy travelling in the opposite direction to the valence electrons is called a hole. For convenience holes are regarded as positive charge carriers. Detailed descriptions of conduction processes in semiconductors can be found in Streetman (1990).
The most commonly used semiconductor material is Silicon, which is tetravalent and forms a diamond type crystal structure. The minimum energy required to promote an electron from the valence state (or band) to the conduction band in silicon is only about 1.1 eV (the energy gap). Hence, the ambient thermal energy at room temperature is sufficient to produce a low but significant number of conduction electrons. Even though the mean thermal energy of the electrons is less than the energy gap there are always some individual electrons with greater energies due to the statistical distribution in the electron energies.
Doping is the addition of minute amounts of specific impurities to extremely pure semiconductor materials to build in a number of available carriers. In materials that are called p-type trivalent atoms like boron have been added to create holes. Whereas, introducing a pentavalent impurity like phosphorus produces n-type material with a background level of conduction electrons. It should be noted that the material is still electrically neutral. The name n or p type describes the type of free carrier that predominates in the material (called the majority carrier). In general both electrons and holes are present. Further information regarding doped semiconductors and device fabrication may be found in Pulfrey and Tarr (1989).
Streetman, B. G. (1990) Solid State Electronic Devices, 3rd edn., Prentice Hall International.
Pulfrey, D. L. and Tarr, N. G. (1989) Introduction to Microelectronic Devices, Prentice Hall-International.