Semi-conductors are found to be much like insulators in that as temperature increases so does the conductivity. In general semi-conductors are found to have higher conductivity than insulators and lower conductivity than metals. The reason that the conductivity is greater in semi-conductors is that the band gap is smaller than that found in insulators. It could even be said that insulators are really just semi-conductors with poor conductivity. Below is a diagram of a semi-conductor in relation to an insulator.
As seen in the image, the band gap is indeed smaller, with the Fermi Level located approximately in the middle of the band gap. Therefore, with thermal excitation, electrons are able to get into the conduction band, resulting in a current. Since the band gap is smaller than that of an insulator, there is an increase in conductivity relative to the insulator under thermal excitation of the electrons. The best known semi-conductor is silicon.
It is possible to increase the conductivity of a semi-conductor by implanting foreign atoms into an otherwise pure material. This is referred to as doping, and the atoms used are dopants. This results in two different kinds of semi-conductors: n-type and p-type, so called for the charge carrier involved. These will both be discussed in a separate section.
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