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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).
The diffusion capacitance is capacitance of a diode when small forward bias is applied ie.,$V_f (\text{forward voltage})<V_{\gamma} (\text{cut in voltage})$. So as the diode is forward biased increases the capacitance increases according to the formula up to cut in voltage, after that the diode barrier breaks and starts conducting current hence the capacitance drop. Note:- $V_{\gamma}$for silicon is $0.7$ volts & for germanium it is $0.3$ volts.
