Drift current Is of PN junction under open-circuit conditions

[anhnha]

Hi,
I need help to understand this section from my book.

Here we are considering drift current Is of PN junction under open-circuit conditions.
I don't understand why the drift current Is is indepentent of the value of the depletion layer voltage Vo.
I think the voltage create an electric field that helps minority charges in N, P materials establishing drift current Is. Therefore, it is of course dependent on Vo.
Can you tell me where I am wrong here?

 

[mfb]

I think it is assumed that all generated minority carriers reach the junction. The generation process does not depend on the voltage.