# Q & A: depletion regions

Most recent answer: 02/02/2013
Q:
hey..... again with a doubt ............. i am asking how is it formed... the potential barrier or the depletion region (if they r same....)in the context of the semiconductors and their p-n junction... i mean if there are just holes that are to be occupied by the electrons then it become s nothing more than a electron..... the same it was already......... and they are already in the conduction band so available for flow as a current..... if the middle part is the steady electrons(the electrons that have occupied a hole in the center of the junction.....) and the holes.... then with the help of the potential difference that is being provided is it not that they should move to occupy the other holes available hence leading to easy flow os current even in reverse biased........ .......... well if i could not explain it clearly ... sorry .... but still thanks for ur time..... thanks
- apurva (age 17)
A:
I think I know what you're asking about. One region is n-doped, with fixed positive charges and free electrons. Near it there's a p-doped region, with fixed negative charges and free holes. In between there's a depletion region with no fixed charges and very low electron-hole concentrations.

I guess you're asking why electrons don't just wander from the n region into the depletion region. If a very small number do, that leaves behind a net positive charge on the n region, which holds the other electrons in via the electrostatic potential. The same argument works, with opposite sign, to explain how the p region holds on to almost all its holes.

The electrons and holes that do make it into the depletion region for the most part annihilate, leaving only the very low intrinsic thermodynamic equilibrium concentration of electrons and holes.

If an external voltage is applied, current can flow easily in the direction which brings electrons in from n and holes in from p, to annihilate in the middle. In the opposite direction which is essentially trying to pull holes and electrons out of the depletion region, the rate at which they spontaneously form is so low that very little current flows.

Mike W.

(published on 04/29/2010)

## Follow-Up #1: what is a depletion region?

Q:
what is depletion region?
- Md.Mizanur Rahman (age 21)