A semiconductor is a material which has electrical conductivity between that of a metal and an insulator. Semiconductors have a number of unique properties. They can change the conductivity by the addition of impurities (doping) or by the application of electric fields or light. This ability makes semiconductors very useful for devices that amplify, switch or convert electrical energy. The study of semiconductors and their properties relies on quantum physics to explain the motion of electrons inside a lattice of atoms.
Semiconductors are the foundation of modern solid state electronics. This includes radios, computers and telephones. Semiconductor-based electronic components include transistors, solar cells, light-emitting diodes and digital and analog integrated circuits. The increase in understanding of semiconductor materials has allowed the continuing increase in the complexity and speed of semiconductor devices.
A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through different techniques, the semiconductor can be modified to have excess of electrons or a deficiency of electrons. In both cases the semiconductor will become more conductive.
When the semiconductors are doped they join to metals, different semiconductors, and to the same semiconductor with different doping. The result of this junction often strips the electron excess or deficiency out from the semiconductor near the junction. The depletion region is rectifying and used to further shape electrical currents in semiconductor devices.
Electrons can be excited across the energy band gap of a semiconductor by various methods. These electrons carry their excess energy over a distance before dissipating their energy into heat. This effect is essential to the operation of bipolar junction transistors.
Electrons in semiconductors will absorb light and retain the energy from the light for a long enough time to be useful for producing electrical work instead of heat. This effect is used in the photovoltaic cell. Semiconductors can use thermoelectric generators to convert temperature differences into electrical power and vice versa. Peltier coolers use semiconductors for this reason.