1. A p–n junction is a boundary or interface between two types of semiconductor material, 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. 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.
2. When a p-type semiconductor is brought into a close contact with n-type semiconductor crystal, the resulting arrangement is a PN junction or junction diode. On account of difference in concentration of charge carriers in the two sections, the electrons from n-region diffuses through the junction into p region and the holes from p-region diffuse into n-region. Due to this the electron falls into the vacancy i.e., it completes the covalent bond. This process is called electron-hole recombination. As a result of the migration of charge carriers across the junction, the electrons leave ionised donor atoms which are bound and cannot move. Similarly, the p-region of the junction will have ionised acceptor atoms which are immobile.
https://en.wikipedia.org/wiki/P%E2%80%93n_junction (1.); http://www.tutorvista.com/content/physics/physics-iv/semiconductor-devices/p-n-junction.php (2.)
Author: Simon Waterstradt