depletion region
/ dih-PLEE-shun REE-jun /
Imagine two neighborhoods, one full of wandering electrons and one full of empty seats. Right at their border, electrons stroll across and settle into the nearest empty seats, until the strip along the border has no wanderers and no empty seats left — just a quiet, swept-clean no-man's-land. That cleared strip is the depletion region.
More precisely, the depletion region is the thin layer at a p-n junction from which mobile carriers have been removed. On the n-side near the boundary, donor atoms have given up their electrons and are left as fixed positive ions; on the p-side, acceptor atoms hold extra electrons and are left as fixed negative ions. These exposed, immobile charges set up an internal electric field across the layer — strong enough to stop further carriers from crossing, which is why the region stays depleted.
The depletion region is where a junction's special behavior actually lives: its width responds to applied voltage, narrowing when you push current the easy way and widening when you reverse it, which is exactly how diodes block current and how transistors switch. A subtle point: the region is not literally empty of atoms — the crystal lattice is all still there. What is missing is the mobile carriers; only fixed ionized atoms remain to hold the field.
In a typical silicon diode the depletion region is only about a millionth of a metre wide, yet the entire built-in voltage of the junction is concentrated across this sliver. Applying a reverse voltage widens it; that widening is what a varactor diode exploits to act as a tunable capacitor.
The depletion region is thin, but it holds the junction's entire built-in field.
The depletion region is also called the space-charge region, because it is the one place in the junction that carries net charge — from the exposed fixed ions — even though the material on either side is neutral.