A semiconductor is a material which has an electrical conductivity value
falling between that of a conductor, such as copper, and an insulator, such as
glass. Its resistivity falls as its temperature rises; metals behave in the
opposite way. Its conducting properties may be altered in useful ways by
introducing impurities ("doping") into the crystal structure. When two
differently doped regions exist in the same crystal, a semiconductor junction
is created. The behavior of charge carriers, which include electrons, ions,
and electron holes, at these junctions is the basis of diodes, transistors,
and most modern electronics.