CarrierDepletion

Carrier depletion is the reduction of free charge carriers (electrons and holes) in a region of a semiconductor caused by electric fields that remove or repel these carriers. The resulting area is a depletion or space-charge region where mobile carriers are scarce, and the region is populated instead by fixed ionized dopants. This depletion lowers the local conductivity and creates an electric field that opposes further diffusion of carriers.

In a typical p–n junction, diffusion of carriers across the junction leaves behind positively charged donor ions and negatively charged acceptor ions near the interface. These fixed charges establish a built-in electric field and a depletion region on both sides of the junction. The width of this region depends on the doping concentrations and any externally applied bias: under reverse bias the depletion width increases, widening the region and reducing current, while under forward bias the region narrows, allowing greater current flow. The depletion approximation is a common modeling approach that assumes negligible mobile carriers within the region and attributes the field to the fixed charges.

Depletion also occurs in other semiconductor interfaces, such as metal–semiconductor contacts and metal-oxide-semiconductor (MOS) structures. In MOS devices, applying a gate voltage can deplete carriers from the semiconductor surface, creating a low-conductivity region that modulates device capacitance and current. Depletion regions play a central role in the behavior of diodes, photodetectors, transistors, and sensors, influencing breakdown voltages, response times, and switching characteristics.

Measurement of depletion width commonly uses capacitance–voltage profiling or device I–V characteristics, and precise control of depletion is essential in semiconductor design and operation.