A semiconductor material has an electrical conductivity value falling between that of a conductor, like copper, gold, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behaviour opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities into the crystal structure. Some examples of semiconductors are silicon, germanium, and gallium arsenide. Silicon is a critical element for fabricating most electronic circuits.