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.