Electronic Devices - Semiconductor Diodes - Discussion

Diffusion capacitance is due to the minority carriers that get stored in the depletion region (as the depletion width during forward bias is less, the capacitance will be high). And the transition capacitance is the rate of change of charge with respect to voltage. So the diffusion capacitance is high during forward bias.

When pn junction diode connect in forward bias it means connect to battery.

Holes from the p-region move toward to negative terminal of battery. Electrons from n-region move towards to positive terminal of battery.

So mostly they combine (diffuse) near to junction so make diffusion layer. And work like capacitor. When pn connect to reverse bias.

Holes from the p-region move toward to negative terminal of battery. Electrons from n-region move towards to positive terminal of battery.

Holes and electrons move away from the junction. So become gap near junction. Means not combine near junction and made up depletion layer.

When diode is reverse bias transition capacitance is appear while in forward bias diffusion capacitance is appear. In reverse bias majority of carrier flow away from the junction and accumulate at the boundary of diode width of capacitance is more so transition capacitance is less.

But in forward bias minority carrier is flow away from the junction and accumulate. Width of depletion region is less so always and always depletion capacitance is greater than the transition capacitance.

What is diffusion and depletion capacitance?

In forward bias, diffusion capacitance dominates. While in reversed bias, depletion capacitance dominates.