Semiconductors
You know that metals are good conductors of electricity. Substances like glass, plastic, porcelain are insulators. Some substances like germanium and silicon are neither good conductors nor insulators. Their conductivity lies in between that of conductors and insulators. Such substances are called semiconductors.
Conductivity of semiconductors : you know that the atoms of silicon and germanium have four electrons each in their outermost shell. All these electrons are involved in covalent bonding and are not free. Hence these elements should behave like insulators.
However in these cases, usually a small amount of energy (in the form of heat or light) is sufficient to break a covalent bond and set and electron free to move. Thus even at room temperatures, a substantial number of electrons will be dissociated from their atoms and this number increases with temperature. This is the reason for these elements exhibiting conductivity.
Further, when an electron is removed from a covalent bond, it leaves a vacancy, equivalent to positive charge. An electron from a neighbouring atom, can drop into this vacancy, leaving the neighbour with a vacancy. In this way, the vacancy usually called a ‘hole’ can move and serves as an additional ‘charge carrier’
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