The term ‘semiconductor’ refers to a solid body whose electrical conductivity is between that of a conductor and that of an insulator.
If the temperatures are very low, the behaviour of a semiconductor is very similar to that of an insulator.
Semiconductors can be monoatomic crystals such as germanium and silicon or compounds with gallium arsenide. These semiconductors are particularly important because their electrical properties can be adjusted in many ways by adding (doping) very small amounts of impurity atoms. Semiconductors can be ultra-cleaned by means of plasma and equipped with a very fine structure by means of plasma etching with a photoresist mask.