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between these two major categories is a class of materials called semiconductors. as the name implies, a semiconductor is a material with conductivity roughly midway between conductors and insulators. it is not just a poor conductor; it has two important properties. first, its resistance normally decreases with increase of temperature, while in conductors resistance slightly increases with temperature. second, the flow of current in a semiconductor may be caused either by a flow of negative electrons or by a movement of missing sites in the opposite direction. if an atom has one outer electron missing, a loosely bound electron from a neighbouring atom can jump into it, leaving behind a new vacant site; this, in turn, can be filled by an electron from the third atom and so on. the impression is as if the vacant site has moved. such vacant sites are called “holes” and since a negative electron is missing, the hole can be said to have a positive charge.