Diode Capacitance

DIODE CAPACITANCE

Diode offers two types of capacitance, one in forward bias and another one in reverse bias. They are,

1. Diffusion or Storage capacitance

2. Depletion or Transition capacitance

1. Diffusion or Storage capacitance

The capacitance which exists in a forward biased junction is called a diffusion or storage capacitance. It is called diffusion capacitance to account for the time delay in moving charges across the junction by diffusion process.

If in a forward-biased junction the applied voltage is suddenly reversed, then forward current Ip ceases at once but a lot of majority charges carries are left in the depletion region. This charge represents a stored charge in the reverse bias condition and should be removed from the space charge region. This removal of charge takes a finite time. This effect is similar to the discharge of a capacitor. Therefore, the amount of stored charge repersents the magnitute of diffusion capacitance. It may be expressed as

where C_D = Diffusion capacitance

η = Constant (η = 2 for Si, η = 1 for Ge)

VT = Volt equivalent of temperature

r = Mean life time of carriers

IF = Forward current

Hence, the diffusion capacitance is directly proportional to the value of forward current IF. The typical value of diffusion capacitance is about 0.02μF. The effect of diffusion capacitance is negligible for a reverse-biased PN-junction.

2. Depletion or Transition Capacitance

When a PN-junction is formed, there exists a depletion region at the junction. This depletion region or layer consists of positive and negative immobile ions. This depletion layer is non-conductive and hence acts as a dielectric medium between P- region and N- region. The P-region and N-region act as the two plates of a capacitor because they have a low resistance. These two plates are separated by a dielectric (depletion Layer).

The capacitance of a parallel plate capacitor is

where ∈ is the permittivity of dielectric

A is the area of plates and d is distance between plates.

Since the depletion layer width (d) increases with increases in reverse bias voltage, the depletion capacitance should decrease with increase in reverse bias. In other words, the depletion capacitance dominates in reverse bias.

Depletion capacitance is given as

Where K is a constant.