I'm confused on the topic of diffusion capacitance in a diode. In the book, Solid State Devices by Streetman and Banerjee, I see an equation for the diffusion capacitance which gives an exponential dependence on voltage. However, there is also this figure:
I do not understand why the diffusion capacitance drops so suddenly in the figure. It doesn't seem to be captured by the equation given: C ~ exp(qV/KT).