Formation of a p-n Junction: The Depletion Region

Bringing p-type and n-type together. A p-n junction is formed when a p-type region and an n-type region exist within a SINGLE, continuous semiconductor crystal (produced, in practice, by changing the type of dopant introduced during crystal growth, never by physically gluing two separately grown pieces together, which would leave a defective, poorly conducting interface).

The initial diffusion of carriers. At the moment of formation, the p-side has a huge concentration of majority holes and almost no electrons, while the n-side, right next door, has a huge concentration of majority electrons and almost no holes. This sharp concentration difference, right across the junction, drives a brief process of DIFFUSION: some free electrons from the n-side diffuse across into the p-side (where they promptly recombine with the abundant holes there), and some free holes from the p-side diffuse across into the n-side (where they recombine with the abundant electrons there).

Formation of the depletion region. Every recombination event right at the junction removes BOTH a mobile electron and a mobile hole from circulation, leaving behind, in a thin layer on EACH side of the junction, only the fixed, immobile dopant ions that Section 9.8 described: negative acceptor ions exposed on the p-side edge of this layer, and positive donor ions exposed on the n-side edge. This thin layer, now stripped of essentially all its mobile charge carriers and containing only these fixed ions, is called the depletion region (or depletion layer, or space-charge region). …