Semiconductors only have a small number of delocalised electrons that are able to act as charge carriers.Many atoms close together cause the energy levels of electrons to become much wider and become energy bands.Electrons in the valence band are tied to the atoms and not available to conduct. Electrons that have enough energy will jump to the conduction band, become delocalised and can move through the semiconductor.Semiconductors have fewer delocalised conduction electrons than conductors.As the temperature rises more electrons are promoted to the conduction band and the resistivity decreases.When an electron moves away it leaves a "hole" in the atom and the atom becomes positively charged.Another electron from another atom can move to fill the hole. The holes therefore appear to move in the opposite direction to the electrons.