pnJunction

A PN junction is the boundary formed when p-type and n-type regions of a semiconductor are joined. The p-side has holes as the majority carriers, while the n-side has electrons. When the two regions come into contact, carriers diffuse across the interface, leaving behind fixed ionized dopants and creating a depletion region. This region is depleted of free charge carriers and is accompanied by an electric field and a built-in potential that oppose further diffusion in equilibrium.

The depletion width depends on the dopant concentrations NA and ND, the semiconductor permittivity ε, and the potential across the junction. In the depletion approximation, the total width W is proportional to sqrt((ε/q) (NA+ND)/(NA ND) (V_bi − V)). The junction forms a capacitance Cj = ε A / W, which varies with bias.

Electrical behavior: The PN junction behaves as a diode. Under forward bias, the potential barrier is reduced, allowing carriers to cross the junction and producing an exponential current I ≈ I_S (exp(qV/kT) − 1). Under reverse bias, the current is small and nearly constant until breakdown occurs. Real devices exhibit leakage, series resistance, and breakdown mechanisms such as Zener or avalanche breakdown at higher reverse voltages.

Applications and materials: PN junctions are used in rectifiers, demodulators, and switching devices, and in solar cells and light-emitting diodes as active junctions. Silicon is the most common material, but junctions are also made in germanium, gallium arsenide, and silicon carbide. Manufacturing methods include diffusion or ion implantation to create abrupt p-n interfaces, followed by annealing to activate dopants. The concept of the PN junction dates to early diode work by Russell Ohl in 1939.