When Pin diode is no longer junction?
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.