Semiconductors are materials which have a intermediate conductivity between conductors (generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors act as an insulator at low temperatures and are electrically conductive at room temperature. Semiconductors can be pure elements, such as silicon or 

germanium, or compounds such as gallium arsenide or cadmium selenide. Semiconductors can change when there are impurities, different impurities can create different semiconductors, this process called doping. Where two different semiconductors are bonded together, it creates a contact layer (semiconductor junction). The properties of charge carriers such as electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics.  

Semiconductor devices

Semiconductor devices can display a range of useful features such as passing current more easily in one direction than the other, showing variable resistance, and sensitivity to heat or light. Because the electrical properties of a semiconductor material can be modified by doping, or by the application of light or electrical fields, devices made from semiconductors can be used for switching, amplification, and  energy conversion.