Depletion region

In semiconductor physics, the depletion region, also called depletion layer or depletion zone, as well as the junction region or the space charge region is an insulating region within a conductive, doped semiconductor material where the charge carriers have diffused away or have been swept away by an electric field. Understanding the depletion region is key to explaining modern semiconductor electronics: the operation of diodes, bipolar junction transistors, field effect transistors, and variable capacitance diodes rely on depletion region phenomena.

Conceptual overview

The depletion region forms across the P-N junction when the junction is in thermal equilibrium, i.e. things are in a steady state.

Electrons and holes will diffuse into regions with lower concentrations of electrons and holes, much as ink will diffuse into water until it is uniformly distributed throughout. N-type semiconductor has an excess of free electrons, and P-type has an excess of holes. Therefore when N-doped and P-doped pieces of semiconductor are placed together to form a junction, electrons will diffuse into the P side and holes will diffuse into the N side. However when a hole and an electron come into contact, they eliminate each other through recombination. This bares the donor atoms adjacent to the depletion region, which are now charged ions. The ions are positive on the N side and negative on the P side, creating an electric field that counteracts the continued diffusion of charge carriers. When the electric field is sufficient to repel incoming holes and electrons, the depletion region reaches its equilibrium width. Integrating the electric field in the depletion region gives what is known as the built-in potential (also called the junction voltage or barrier voltage).

Under reverse bias (P negative with respect to N) this potential is increased, further widening the depletion zone. Forward bias (P positive with respect to N) narrows the zone and eventually reduces it to nothing, making the junction conductive and allowing free flow of charge carriers.

The depletion region is so named because it is void of all majority carriers. In other words, the recombination of holes and electrons at the P-N junction causes the region to become depleted of mobile charge.