P–n junction

A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electric current to pass through the junction only in one direction. The p- and n-type regions creating the junction are made by doping the semiconductor, for example by ion implantation, diffusion of dopants, or by epitaxy (growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant). p–n junctions are elementary "building blocks" of semiconductor electronic devices such as diodes, transistors, solar cells, light-emitting diodes (LEDs), and integrated circuits; they are the active sites where the electronic action of the device takes place. For example, a common type of transistor, the bipolar junction transistor (BJT), consists of two p–n junctions in series, in the form n–p–n or p–n–p; while a diode can be made from a single p-n junction. A Schottky junction is a special case of a p–n junction, where metal serves the role of the n-type semiconductor. The invention of the p–n junction is usually attributed to American physicist Russell Ohl of Bell Laboratories in 1939. Two years later (1941), Vadim Lashkaryov reported discovery of p–n junctions in Cu2O and silver sulphide photocells and selenium rectifiers. The modern theory of p-n junctions was elucidated by William Shockley in his classic work Electrons and Holes in Semiconductors (1950). The p–n junction possesses a useful property for modern semiconductor electronics. A p-doped semiconductor is relatively conductive. The same is true of an n-doped semiconductor, but the junction between them can become depleted of charge carriers, depending on the relative voltages of the two semiconductor regions.