Junction capacitance is the capacitance which forms in a PN junction diode under reverse
bias. When reverse bias voltage is applied across a PN junction diode, the two regions P
and N behave as electrodes (with less resistance) and the depletion region between two
regions acts as dielectric medium (with more resistance). The P., N regions (electrodes)
and depletion region (dielectric medium) allows the storage of charge in electric field.
This capacitance thus formed is termed as junction capacitance or transition capacitance
or depletion capacitance.