Difference between diffusion capacitance and depletion/transition capacitance

The reactance of a capacitor is given by Xc=1/2*pi*f*C. At lower frequency, Xc is very very large and we can treat it as open circuit. However, at high frequencies, the Xc value becomes smaller and significant and we will not be in the position to ignore. While dealing with the electronic devices at higher frequencies, two capacitance come into picture. They are

1) Diffusion Capacitance
2) Depletion Capacitance, also called as transition capacitance.

Difference between diffusion capacitance and depletion/transition capacitance

1. Depletion Capacitance

Depletion capacitance will be dominant  in reverse bias region.

The capacitance of a parallel plate capacitor is given by C =εA/d.

Under reverse bias condition, the depletion region acts as  a parallel plate capacitor with the depletion region width as d, and it's effective area as A in the above equation.

Depletion width(d) increases when the reverse bias voltage increases, so the depletion capacitance  decreases with increase in reverse bias voltage.

2. Diffusion Capacitance

Diffusion Capacitance will be dominant in forward bias condition.

CD = τID / ηVT where VT=KT/q

The diffusion capacitance decreases with decreasing current and increasing temperature.


Note: Both this capacitance will come into picture at higher frequencies and one can ignore them at lower frequencies.