In a p-n junction diode, two types of capacitance take place. They are,
Transition capacitance (CT)
Diffusion capacitance (CD)
1. Transition capacitance (CT)
We know that capacitors store electric charge in the form of electric field. This charge storage is done by using two electrically conducting plates (placed close to each other) separated by an insulating material called dielectric. The conducting plates or electrodes of the capacitor are good conductors of electricity. Therefore, they easily allow electric current through them. On the other hand, dielectric material or medium is poor conductor of electricity. Therefore, it does not allow electric current through it. However, it efficiently allows electric field.
When voltage is applied to the capacitor, charge carriers starts flowing through the conducting wire. When these charge carriers reach the electrodes of the capacitor, they experience a strong opposition from the dielectric or insulating material. As a result, a large number of charge carriers are trapped at the electrodes of the capacitor.
The transition capacitance can be mathematically written as,
CT = ε A / W
ε = Permittivity of the semiconductor
A = Area of plates or p-type and n-type regions
W = Width of depletion region
2. Diffusion capacitance (CD)
Diffusion capacitance occurs in a forward biased p-n junction diode. Diffusion capacitance is also sometimes referred as storage capacitance. It is denoted as CD. In a forward biased diode, diffusion capacitance is much larger than the transition capacitance. Hence, diffusion capacitance is considered in forward biased diode. The diffusion capacitance occurs due to stored charge of minority electrons and minority holes near the depletion region. When forward bias voltage is applied to the p-n junction diode, electrons (majority carriers) in the n-region will move into the p-region and recombines with the holes. In the similar way, holes in the p-region will move into the n-region and recombines with electrons. As a result, the width of depletion region decreases.
The formula for diffusion capacitance is
CD = dQ / dV
CD = Diffusion capacitance
dQ = Change in number of minority carriers stored outside the depletion region
dV = Change in voltage applied across diode