A solid state material that can be electrically altered. Certain elements in nature, such as silicon, perform like semiconductors when chemically combined with other elements. Various optical materials can also change their state (see phase change disc).

When electricity or light is applied to semiconductors, they change their state between conductive and non-conductive or reflective and non-reflective. The most significant semiconductor is the transistor, which in digital circuits works like an on/off switch. For analog applications, it may be an on/off switch as well, but is more likely used as an amplifier, taking in a low-voltage signal and outputting a higher voltage. See n-type silicon, doping and chip.