What is a Junction capacitance of a diode?

The increase or decrease of uncovered charge at the junction in depletion layer region can be considered a capacitive effect and is termed as junction capacitance. On the either side of depletion region the combination of charged ions (acceptor atoms in p-region by accepting diffused electrons from n-region becomes positively charged and the donor atoms by accepting the diffused holes from p-type region becomes positively charged) acts as a parallel plate capacitor. Any variation in the applied voltage causes a corresponding change in charge stored in the depletion region, Hence the junction capacitance is given as Cj = dqj / dvj where dqj is variation in charge stored in depletion region due to variation in voltage applied across junction dvj.