Why does the the diffusion capacitance fall off at high frequencies?

1. At high frequencies the dopants are vibrated across the metallurgical junction and the doping profiles smooths out.
2. Since Gd increases with frequency Cd must decrease with frequency and the parallel combination of Gd and Cd is constant.
3. When the applied bias frequency is higher than the speed which the majority carriers can respond the diode starts to disappear electrically.
4. It takes time to store and remove minority charge, so when the frequency is higher than the inverse of the minority carrier lifetime the carriers can't respond as well.