# When Pin diode is no longer junction?

For p-i-n junction with intrinsic region d, I need to answer what happens when d is very large.
So from simple logic, I believe that at some point there will be no diffusion of charge carriers from p-type to n-type and reverse and I guess that these relate to diffusion length, but the diffusion length is more than 100 μm so that cannot be the reason.

Any idea?

• The '0 down vote favorite' bit comes from one copying a thing from another posted question or answer on SE. @Bort – Richard the Spacecat Jun 28 '18 at 18:46

Consider that there isn't any truly perfect intrinsic material with perfectly balanced donor and acceptor impurities. Any real p-i-n junction is really either a $p-p^--n$ or $p-n^--n$ junction.

So if you make the i region too long you'll end up with effectively either a $p-n^-$ junction or a $p^--n$ junction that just happens to have a more heavily doped region away from the junction on the lower doped side.

• Yeah, my guess is that the carriers will just be fully captured by the "intrinsic" region and you get a depletion region that's kind of pushed towards the p^- - n or n^- p side. – hatsunearu Jun 30 '18 at 14:08

Since Silicon has a breakdown of 10V to 100V per micron , I expect this to in increase the Avalanche voltage according to gap, but also increase diffusion lifetime and collection probability such as in Solar cells.