PN junction

A PN junction is the boundary where a slice of semiconductor doped with extra holes (p-type) meets a slice doped with extra electrons (n-type) — and it is the single most important place in all of electronics, because almost every diode, transistor and solar cell is built from one or more of them. Picture two crowds pressed together at a doorway: electrons from the n-side spill across to fill holes on the p-side, leaving behind a thin no-man's-land near the boundary that has been swept clean of free charge.

That stripped region, called the depletion region, holds fixed charged atoms that set up an internal electric field — like a built-in battery of about 0.6–0.7 V in silicon — that opposes further crossing. Push current the 'easy' way (forward bias) and you cancel this field so charge floods across; push the other way (reverse bias) and you widen the barrier so almost nothing flows. This one-way valve behaviour is the root of rectification.