A p-n junction is made by joining P-type and N-type semiconductors together in extremely close contact. The word junction consider to the boundary interface in which the 2 regions of the semiconductor meet. If they were constructed of two separate pieces this would bring in a grain boundary, thus p-n junctions are formed in a single crystal of semiconductor by doping, for instance by ion implantation, diffusion of dopants, or through epitaxy (growing a layer of crystal doped with one kind of dopant on top of a layer of crystal doped with another sort of dopant).
P-N junctions are elementary "building blocks" of approximately all semiconductor electronic devices like diodes, transistors, solar cells, LEDs, and integrated circuits; they are the active sites in which the electronic action of the device occurs. For instance, a common sort of transistor, the bipolar junction transistor, contains two p-n junctions in series, in the make n-p-n or p-n-p.
Generally, p-n junctions are manufactured from a single crystal with dissimilar dopant concentrations diffused across it. Forming a semiconductor from two separate pieces of material would introduce a grain boundary among the semiconductors that severely inhibits its utility by scattering the electrons and holes. Though, in the case of solar cells, polycrystalline silicon is frequently employed to reduce expense, despite the lower efficiency.