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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?

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

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  • \$\begingroup\$ 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. \$\endgroup\$ – hatsunearu Jun 30 '18 at 14:08
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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.

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