Junction  capacitance

Some of the P-N junctions can alternate between conduction (in forward bias) and nonconduction (in the reverse bias) millions or billions of times every second. Other junctions are slower. The basic limiting factor is capacitance at P-N junction during the conditions of reverse bias. The amount of capacitance depends on many factors, including the operating voltage, type of semiconductor material, and the cross sectional area of P-N junction.

By examining the figure B one should notice that the depletion region, sandwiched between 2 semiconducting sections, resembles dielectric of the capacitor.  Actually, the similarity is such that a reverse biased P-N junction is a capacitor. Some semiconductor components are made with this property in mind.

The junction capacitance can be varied by changing reverse bias voltage, as this voltage affects the width of depletion region. The greater the reverse voltage, the extensive the depletion region becomes, and smaller the capacitance becomes.