When a material is neither a good conductor nor a good insulator it is called a semiconductor. Silicon is one of the most commonly used elements in devices that require superconductors. It is a semiconductor because it forms crystals where each of the valence electrons is shared with one other silicon atom. While the resulting Silicon lattice may look metallic the lack of free electrons makes it unlike a metal in that it does not conduct electricity well. This semiconductor can be doped with Boron or Phosphorous, which causes it to become a P-type or N-type semiconductor. An N-type is made by doping with Phosphorous, giving the semiconductor some free electrons, which can flow through the material. Having extra electrons it is more negative ('N' in N-type stands for negative). P-type is made by Boron doping, which gives it empty 'holes' which like the electrons can flow. The 'holes' make it more positive, giving the 'P' in P-type. A similar semiconductor setup with free electrons and holes is GaAs (Gallium-Arsenide).



These semiconductors can be used in two ways, as diodes and transistors. A diode is made of one layer of N-type and one layer of P-type, which put together let electricity flow in one direction but not the other.



Transistors are made up of three alternating layers, 'NPN' or 'PNP'. When a small current is run through the center it becomes a good conductor of electricity. This behavior causes it to be able to be used as a switch or amplifier.