Why is the diffusion capacitance is negligible for a reverse biased diode?

The diffusion capacitance in a diode is associated with the storage of minority carriers, which occurs primarily under forward bias. In reverse bias, the depletion region widens, and the majority carriers are pulled away from the junction, minimizing the injection and storage of minority carriers. As a result, the diffusion capacitance becomes negligible because there is insufficient minority carrier recombination and storage in this condition. Thus, the behavior of the diode under reverse bias is dominated by junction capacitance rather than diffusion capacitance.