pnjunktioonide

Pn junctions (pn-junctions) are the boundary between p-type and n-type semiconductors in a single crystal, formed by diffusing dopants or by epitaxial growth. When the two regions are brought into contact, free carriers diffuse across the interface: electrons from the n-side move into the p-side and recombine with holes, leaving behind charged dopant ions. This creates a depletion region where mobile charge carriers are scarce and a built-in electric field that opposes further diffusion, establishing a built-in potential across the junction.

Under zero external bias, the junction exhibits a small diffusion current due to minority carriers. Under forward bias, the depletion region narrows, enhancing carrier injection and causing an exponential increase in current, described by the diode equation I = I_s (exp(V/(n V_T)) − 1), where I_s is the saturation current, V_T = kT/q, and n is the ideality factor (typically ~1–2). Under reverse bias, the depletion region widens and the current remains very small until breakdown mechanisms such as Zener or avalanche breakdown occur at high reverse voltages.

Key electrical properties include the junction capacitance, which varies with bias as the depletion width changes, and the response time set by minority-carrier lifetimes. Temperature affects I_s and V_bi, generally increasing I_s and reducing the built-in potential with rising temperature.

Applications of pn junctions are broad and foundational: diodes and rectifiers, light-emitting diodes (LEDs) and laser diodes, photodiodes and solar cells, and various signal-processing and rectification roles. Variants include homojunctions (same material) and heterojunctions (different materials), as well as engineered dopant profiles for tailored performance.