The depletion region (also called the space-charge region, SCR) is the thin layer straddling a PN junction that has been emptied of mobile carriers, leaving only fixed ionised dopant charge. It is where the junction’s built-in field lives, and its behaviour under bias is the mechanism behind every diode and transistor.
How it forms
Put p-type silicon next to n-type silicon in the same crystal. The n-side has a huge surplus of mobile electrons; the p-side a huge surplus of mobile holes. The concentration gradient drives diffusion: electrons spill from n into p, holes spill from p into n (this is the diffusion half of Drift and diffusion current).
Now the key step. Each electron that leaves the n-side does not take its compensating positive charge with it — the donor it came from is a fixed ionised donor locked in the lattice. So departing electrons leave behind a layer of immobile, positively-charged donor ions on the n-side. Symmetrically, holes leaving the p-side expose immobile, negatively-charged acceptor ions there. The result is a narrow zone right at the junction containing only fixed ionic charge — positive on the n-side, negative on the p-side — with the mobile carriers swept out. That is why it is “depleted.”
The built-in field reaches equilibrium
That exposed double layer of fixed charge (positive then negative) sets up an electric field pointing from the n-side toward the p-side. This field opposes the diffusion that created it: it pushes electrons back toward n and holes back toward p — i.e. it drives a drift current opposite to the diffusion current. As more carriers diffuse, the exposed charge and the field grow, until the drift current it produces exactly cancels the diffusion current. At that point net current is zero and the junction is in thermal equilibrium. The stable potential difference across the depletion region in equilibrium is the Built-in voltage (about for silicon).
So the depletion region is self-limiting: diffusion creates it, the field it creates stops further diffusion. The equilibrium balance is precisely “Drift and diffusion current cancel.”
Why it controls everything
The width and charge of the depletion region respond to applied voltage, and that response is diode action:
- Reverse bias adds to , pulling even more mobile carriers away and widening the depletion region. The junction blocks current (only the tiny Reverse saturation current from minority carriers crosses).
- Forward bias opposes , narrowing the depletion region and lowering the barrier so a large diffusion current flows.
The exact width is set by the doping through Depletion region width. The same depletion-region physics, with its voltage-dependent width and stored charge, also explains junction capacitance and the operation of the Bipolar junction transistor and MOSFET.