Their conductivity lies between conductor and insulator. Their resistivity is usually high and temperature coefficient of resistance is always negative. These are materials which are having filled energy bands and small forbidden zones. Moving carriers of electric current are originated due to absorption of thermal radiant or electric energy from external source. At low temperature they become dielectrics. They present a rise in conductivity with rise in temperature because of increase in current carriers and vice-versa. These are materials whose valance electrons are bonded somewhat loosely to their atoms. They require energy less then insulators and more than good conductors to remove an electron from their parent atom. They own a vacant conduction band. Approximately filled valance band. Very low energy gap of 1.2ev separating the two bands.