pnjunctie

A p-n junction, sometimes written pnjunctie, is the boundary between p-type and n-type semiconductor regions in a single crystal. It forms when a doped semiconductor with acceptor dopants (p-type) is joined to one with donor dopants (n-type). The junction creates a region where mobile charge carriers diffuse across the interface, leaving behind charged dopant ions.

In thermal equilibrium, diffusion of carriers leaves behind a depletion region free of mobile charges, but with fixed ionized dopants. This depletion region carries an electric field and a built-in potential that opposes further diffusion. At equilibrium the Fermi level is continuous across the junction, and band bending occurs at the interface.

When an external voltage is applied, the depletion width changes. Forward bias reduces the barrier, enhancing carrier injection and producing a significant current. Reverse bias widens the barrier, suppressing current to a small leakage level; at high reverse voltages, breakdown mechanisms may occur. The current in a pn junction is largely due to diffusion of minority carriers and, under bias, drift in the depletion region.

The current–voltage behavior is commonly described by the diode equation I = I_s (e^(V/(nV_T)) - 1) for many models, where I_s is the saturation current, n is the ideality factor, and V_T ≈ kT/q at temperature T. Real devices exhibit non-idealities such as series resistance and breakdown effects.

PN junctions are foundational to many electronic and optoelectronic devices, including diodes (rectifiers), light-emitting diodes, laser diodes, photodiodes, and solar cells. Silicon is the most common material, but other semiconductors like GaAs and SiC are used for specialized applications.