Chapter: Electronic Devices and Circuits : PN Junction Devices

Diffusion capacitance

When the junction is forward biased, a capacitance comes into play , that is known as diffusion capacitance denoted as CD. It is much greater than the transition capacitance.

Diffusion capacitance:

1. When the junction is forward biased, a capacitance comes into play , that is known as diffusion capacitance denoted as CD. It is much greater than the transition capacitance.

2. During forward biased the potential barrier is reduced. The charge carriers moves away from the junction and recombine.

3. The density of the charge carriers is high near the junction and reduces or decays as the distance increases.

4. Thus in this case charge is stored on both side of the junction and varies with the applied potential. So as per definition change in charge with respect to applied voltage results in capacitance which here is called as diffusion capacitance.

5. The formula for diffusion capacitance is CD = τID / ηVT , where τ is the mean life time of the charge carrier, ID is the diode current and VT is the applied forward voltage, and η is generation recombination factor.

6. The diffusion capacitance is directly proportional to the diode current.

7. In forward biased CD >> CT . And thus CT can be neglected.

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Electronic Devices and Circuits : PN Junction Devices : Diffusion capacitance |