Depletion Region
A depletion region is the zone around a pn junction, or beneath a MOS gate, from which mobile carriers have been swept away, leaving only the fixed, charged dopant ions behind. It is a thin slab of crystal that is nearly empty of free electrons and holes yet full of electric field, and it is the working part of diodes, transistors, solar cells and image sensors.
The region forms by itself the moment p-type and n-type material touch, because carriers diffuse down their concentration gradients. Electrons on the n side wander into the p side and holes wander the other way, and wherever they meet they recombine. What they leave behind cannot move: positive phosphorus or arsenic ions on the n side and negative boron ions on the p side. This fixed charge produces an electric field pointing from n to p that pushes the diffusing carriers back, and diffusion stops when the field and the concentration gradient balance exactly. The potential step that results is the built-in potential, about 0.6 to 0.7 volts for typical silicon junctions, and it is roughly the thermal voltage of 0.026 volts times the natural logarithm of the doping product divided by the square of the intrinsic density.
The width of the region depends on the doping and on the applied voltage, and that dependence is what makes it useful. The depletion region extends further into the more lightly doped side, since each side must expose the same total charge, and its width grows roughly with the square root of the total potential across it. At typical doping, widths run from tens of nanometers in heavily doped junctions to several micrometers in lightly doped ones. A reverse bias adds to the barrier and widens the region, while a forward bias subtracts from the barrier and narrows it. Because the two edges of the region are charged plates separated by an insulating slab, the junction behaves as a capacitor whose capacitance falls as reverse voltage rises; a varactor diode is simply a junction optimized for this voltage-controlled capacitance and has long been used to tune radio receivers and oscillators.
Depletion is not confined to pn junctions; the same physics turns a MOS transistor on and off. Under the gate of an n-channel MOSFET on p-type silicon, a modest positive gate voltage first repels holes and uncovers negative acceptor ions, forming a depletion layer, and only a larger voltage pulls in enough electrons to form the inverted channel. The depletion layer beneath the channel also isolates it from the bulk silicon and sets how much charge the gate must supply, which is why doping directly sets the threshold voltage. In image sensors and charge-coupled devices, a depletion region collects the electrons liberated by light, and in a solar cell its field separates photogenerated pairs before they can recombine. Modern FinFETs and fully depleted silicon-on-insulator devices use a thin silicon body that is fully depleted, which gives the gate better control and reduces leakage.
A common misconception is that the depletion region is empty of charge; it is empty of mobile carriers but dense with fixed charge. It is also not a gap or an insulator that blocks current under all conditions: under forward bias it shrinks and current flows, and under strong reverse bias the field inside it can grow large enough to trigger breakdown. Carriers can still be generated thermally inside it, and that generation is the main source of reverse leakage current in a silicon diode. Because the depletion region of one junction is also the region that creates parasitic capacitance, designers in logic chips treat it as both an essential feature and a speed limit, as it contributes to the capacitance that every switching transistor must charge. Junction isolation, an early integrated-circuit technique, also used reverse-biased depletion regions to electrically separate neighboring transistors on one chip.
Read depletion region as the self-built zone at a junction where mobile carriers have left and only fixed ions remain, whose width the applied voltage can open or close.