The depletion layer capacitance is essentially the capacitance of a _________ p-n junction.

The correct answer is option 3.

Types of capacitance in PN junction diode

1.) Transition capacitance (C_T)

• It is also known as depletion layer capacitance. This capacitance exists when the diode is reverse-biased.
• In a reverse-biased p-n junction diode, the p-type and n-type regions have low resistance. Hence, p-type and n-type regions act like the electrodes or conducting plates of the capacitor.
• The depletion region of the p-n junction diode has high resistance. Hence, the depletion region acts like the dielectric or insulating material. Thus, the p-n junction diode can be considered as a parallel plate capacitor.

2.) Diffusion capacitance (C_D)

• Diffusion capacitance occurs in a forward-biased p-n junction diode. Diffusion capacitance is also sometimes referred to as storage capacitance. 
• When the forward bias voltage is applied to the p-n junction diode, electrons (majority carriers) in the n-region will move into the p-region and recombine with the holes. Similarly, holes in the p-region will move into the n-region and recombine with electrons. As a result, the width of the depletion region decreases.
• A large number of charge carriers, which try to move into another region will be accumulated near the depletion region before they recombine with the majority carriers. As a result, a large amount of charge is stored on both sides of the depletion region.
• The accumulation of holes in the n-region and electrons in the p-region is separated by a very thin depletion region or depletion layer. This depletion region acts like dielectric or insulator of the capacitor and the charge stored at both sides of the depletion layer acts like conducting plates of the capacitor.