Diffusion capacitance is present in:

A diode can have two different capacitances:

Diffusion Capacitance:

It is dominant when the diode is forward and is the result of storage charges when forward biased.

The diffusion capacitance is given by the formula:

\({C_d} = \frac{{{I_{DQ}}\times\tau }}{{2{V_T}}}\)   ---(1)

IDQ = Quiescent current of the diode.

τ = Minority carrier lifetime

VT = Thermal voltage

Since the diode current Equation is given as:

\(I_{DQ}=I_S(e^{\frac{qV_f}{kT}}-1)\)    ---(2)

From Equation (1) and (2), we conclude that the diffusion capacitance increases exponentially with forward bias voltage (Vf).

Junction Capacitance:

It is dominant when the diode is reverse biased and is the result of the charge stored in the Depletion layer.

The junction capacitance is given by the formula:

\({C_j} = \frac{{A\epsilon}}{W}\)

W = depletion region width