pn junction: Capacitances

When calculating the total charge for diffusion capacitance (under forward bias), the and , and resulting capacitances, should be added in parallel!!

The reason: you integrate over the whole accumulated charge!

When adding to , it is also in parallel!!

The reason: the two phenomena are independent physically. Under reverse bias, there is no charge accumulation, and so, no diffusion capacitance.

Thus, the branch of this capacitance vanishes.