PNjunctions

PN junctions are interfaces between p-type and n-type semiconductors formed by bringing together materials doped with acceptor and donor impurities. In a pn junction, carriers diffuse across the interface, leaving behind fixed ionized dopants that create a depletion region and an electric field. At thermal equilibrium, diffusion current is balanced by drift current, resulting in a small net current.

Under zero bias, a built-in potential exists across the junction. The depletion region widens with higher doping, and its width decreases under forward bias and increases under reverse bias.

Current–voltage characteristics follow the Shockley diode equation I = Is (exp(V/nVt) − 1) in an ideal junction, where Is is the reverse-saturation current, Vt = kT/q, and n is the ideality factor. In forward bias, the barrier is lowered, allowing minority carriers to inject into the other side and producing an exponential current increase. In reverse bias, current is small and dominated by minority-carrier leakage; at sufficiently high reverse voltage breakdown can occur by Zener or avalanche mechanisms.

Key parameters include the junction capacitance, which varies with bias due to the changing depletion width, and the series resistance of the doped regions. Practical diodes exhibit non-idealities described by the ideality factor and saturation current, as well as reverse-recovery behavior in switching applications.

PN junctions are widely used as diodes for rectification and demodulation, in light-emitting diodes and laser diodes, and in solar cells. They also form fundamental operating elements in transistor technology, where PN junctions create emitter-base and collector-base junctions.