PN Junction

A p-n junction is formed by joining p-type and n-type semiconductors (in a single crystal, not metal contact).

Diffusion: electrons from n-side diffuse to p-side; holes from p-side diffuse to n-side. Diffusion currents flow.

Diffusion leaves IONISED dopants behind — a DEPLETION region (charge-depleted of mobile carriers) forms at the junction.

The ionised donors (positive) and acceptors (negative) create a built-in electric field that OPPOSES further diffusion ⇒ equilibrium.

Built-in potential V₀ (~0.6-0.7 V for Si, ~0.3 V for Ge) is the equilibrium contact potential.

Forward bias (p connected to +): reduces V₀ and depletion width ⇒ large current flows.

Reverse bias (p connected to −): increases V₀ and depletion width ⇒ tiny saturation current.

This asymmetry makes the p-n junction a DIODE — passes current one way, blocks the other.