Semiconductors are electronic devices that, under specific conditions of temperature and voltage, operate as insulators or conductors. They are also called solid-state devices since they do not use vacuum tubes to conduct electricity. These items use electronic conduction in the solid state as opposed to the gaseous state or thermionic emission in a high vacuum. Semiconductor units are made as single devices or as integrated circuits (ICs), which consist of a number devices manufactured onto a single semiconductor substrate. The exact definition is "a solid valence band is at full absolute zero."
In very low temperature the semiconductor will act as an insulator and it will act as a conductor at room temperature, however it has much lower conductivity than a conductor. A semiconductor is different than a conductor as, at absolute zero, the uppermost filled electron energy band is completely filled in a semiconductor and only partially filled in a conductor.
The difference between a semiconductor and an insulator is a little more arbitrary. A semiconductor has a band gap that is small enough so that its conduction band is appreciably thermally populated with electrons at room temperature. Whereas an insulator has a band gap that is too wide for there to be appreciable thermal electrons in its conduction band at the same temperature.
Semiconductors are widely used in modern electronics and are used in one form or another in white goods, electronic instruments, automobiles, space applications, industrial circuits, audio equipment, etc. Common trade names are transistors, diodes, thyristsors, microprocessors, PIN diodes and zener diodes.
Semi-conducting materials are Silicon, Silicon Carbide, Boron Nitride, Gallium Arsenide, Aluminum Arsenide, Silicon Germanium, Aluminum Gallium Arsenide, Diamond, Gallium nitride, Germanium and Indium Phosphide.
These devices are made of PN layers (anode, cathode) that allow them to conduct electricity. They operate on the principle of electrons and holes. Electrons tend to move towards the hole layer and this makes them conductive. Semiconductors are very useful in electronics because their conductivity can be altered when small amounts of impurities (dopants) are added. This is called doping. When you heavily dope a semiconductor you can actually increase its conductivity by a factor greater than a billion. In modern integrated circuits heavily-doped polycrystalline silicon is sometimes used as a replacement for metals.