Semiconductors have a similar band structure to insulators but the band gap is not very large and some electrons have sufficient thermal energy to be promoted up to the empty conduction band. There are two types of conduction mechanism in semiconductors. Electrons promoted into the conduction band are classed as negative charge carriers and would move towards a positive electrode under an applied potential. The holes these electrons leave behind are known as positive holes. These holes move when an electron enters them. Wherever the electron that filled the hole moved from is the new positive hole. The positive holes therefore move in an opposite direction to the electrons. (fig. 7)

Semiconductors can be split into two groups. INTRINSIC and EXTRINSIC semiconductors.

Intrinsic semiconductors are pure materials with the bandstructure already discussed. The number of electrons in the conduction band is determined only by the size of the band gap and the temperature (more electrons with small band gap and high temperature).


Extrinsic semiconductors are materials where the conductivity is controlled by adding dopants with different numbers of valenece electrons to that of the original material. These are discussed in the next section.

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