Do you mean "breakdown" voltage?
It's been a while since I messed with these things, but wouldn't that voltage vary according to the particular device?
There's a "depletion zone" right where the n-doped material meets the p-doped material, and to get current to flow from n to p, you've got to have 7/10ths of a volt for silicon doped material, and 3/10ths for germanium doped material.
I don't know what it would be if they used germanium on one side and silicon on the other, but there it is.
If you ran the current the other way, I guess it would depend on the way the device was made.
A foggy memory . . .