Semiconductors are a group of materials classified according to their
electrical conductivity properties. Other classifications are
conductors and
insulators. In their pure form, semiconductors are known as '
intrinsic'. The addition of certain foreign material, (known as a
dopant), can create an
extrinsic semiconductor. Introduction of a
dopant material creates an excess of either
electrons or '
holes', (depends on
valence of
dopant and
intrinsic materials), within the material thereby increasing the material's
conductivity.
In terms of energy bands of electrons, semiconductors are characterised by the size of the band gap between their valence and conduction bands. Put simply, this gap is the amount of thermal energy an electron requires to jump from the lower energy valence band to the higher energy conduction band in a material. Electrons in the conduction band are free to move within the material, thus contributing the conduction of electricity through the material. The size of the energy gap varies from material to material. An insulator is a material with a large band gap, with consequently fewer electrons in its conduction band. A conductor, on the other hand, has no band gap so electrons are free to move in and out of the conduction band and the material is therefore highly conductive.
Here's an interesting characteristic of semiconducting materials: As you heat up an intrinsic semiconductor, more electrons are able to jump the gap to the conduction band and the conductivity of the material increases. On the other hand, increasing the temperature of an extrinsic semiconductor causes a decrease in conductivity because the effect of the increase in number of electrons inhabiting the conduction band is outweighed by the reduction in conductivity caused by increased vibration of the material's crystal lattice. The latter is the same mechanism which causes the conductivity of conductor materials, such as metals, to decrease with increasing temperature.