In contrast with metals, semiconductors contain only a relatively small number of quasi-free electrons at room temperature. The conductivity of semiconductors is therefore of the order of 10 -5 times as low as that if metals. It can be increased by breaking up the electronic double bonds, which normally exist, between the tetravalent atoms of semiconductors. By the incorporation of impurities, i.e., foreign atoms of higher or lower valence, which function as points of disturbance in the crystal lattice, the conductivity properties of semiconductors can be varied within very wide limits.
Negative conduction occurs as the result of a surplus of electrons.
Semiconductor elements comprising a transition from positive to negative conduction are called semiconductor diodes, while those which comprise two such transitions are called transistors. The transistor is a semiconductor triode. Its three electrodes are respectively called the emitter, the base and the collector. Control of the passage of current between base and collector is thereby achieved. This enables the transistor to be employed for purposes of amplification and the generation of oscillations.