Transition and Diffusion capacitance
1. When P-N junction is reverse biased the depletion region act as an insulator or as a dielectric medium and the p-type an N-type region have low resistance and act as the plates.
2. Thus this P-N junction can be considered as a parallel plate capacitor.
3. This junction capacitance is called as space charge capacitance or transition capacitance and is denoted as CT .
4. Since reverse bias causes the majority charge carriers to move away from the junction , so the thickness of the depletion region denoted as W increases with the increase in reverse bias voltage.
5. This incremental capacitance CT may be defined as
CT = dQ/dV,
Where dQ is the increase in charge and dV is the change or increase in voltage.
6. The depletion region increases with the increase in reverse bias potential the resulting transition capacitance decreases.
7. The formula for transition capacitance is given as CT = Aε/W, where A is the cross sectional area of the region, and W is the width.
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.