<P> 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 electrical current to pass through the junction only in one direction . The p-n junction is created by doping, 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). If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes . </P> <P> p--n junctions are elementary "building blocks" of semiconductor electronic devices such as diodes, transistors, solar cells, 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, 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 ap--n junction, where metal serves the role of the p - type semiconductor . </P> <P> The p--n junction possesses essential properties for modern 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, and hence non-conductive, depending on the relative voltages of the two semiconductor regions . By manipulating this non-conductive layer, p--n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction . Bias is the application of a voltage across ap--n junction; forward bias is in the direction of easy current flow, and reverse bias is in the direction of little or no current flow . </P>

What is the total current at the junction of pn junction diode