# What is the effect of capacitance of a PN junction diode as forward and reverse bias are applied?

What is the effect of capacitance of a PN junction diode as forward and reverse bias are applied?

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

Where,

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

Where,

CD = Diffusion capacitance

dQ = Change in number of minority carriers stored outside the depletion region

dV = Change in voltage applied across diode

## Related questions

What is the total current at the junction of PN junction diode?
Last Answer : Current flow in a pn-junction diode under forward bias. Under the forward bias (p-side is more positive w.r.t. n-side, or n-side is more negative w.r.t. p-side), holes are injected ... p-side , across the depletion region (around the junction, a region depleted of mobile carriers) into the nside ....

Describe the working of a PN junction diode with neat diagrams. Also explain its V-I characteristics.
Last Answer : Working of a PN junction diode:   if an external potential is applied to the terminals of PN junction, it will alter the potential between the P and N-regions. This ... circuit. When the reverse bias input voltage has reached the breakdown voltage, reverse current increases spectacularly.  ...

Explain the avalanche effect that accounts for the reverse breakdown voltage (PIV) of a Diode
Last Answer : PN Junction Breakdown : Electrical break down of any material (say metal, conductor, semiconductor or even insulator) can occur due to two different phenomena. Those two phenomena are 1) Zener ... be applied to develop the same electric field strength (necessary enough to break electrons free)  ...