Question

how a diode can act as a capacitance

Answer 1

A capacitor is any device that can store charge. The particular way by which any device does this gives rise to several types of capacitors. Sometimes the storage of charge is inadvertent and this gives rise to a phenomenon called leakage or stray capacitance as seen in transmission lines and in the insulation bushings of high power machines like transformers, high rating motors etc.What you need to get capacitance is just separation of charges. This is generally caused due to the appearance of an electric field across a dielectric material. The charge appears on the either surface of the dielectric. This is because dielectrics are insulators. In fact, even if you apply a field across vacuum using two conducting plates at some distance, there will be an electric field, and charges will be set up on the two conducting plates.Something resembling this is what happens in a diode.Consider a PN Junction diode.It is said to be reverse biased when the P Region is provided a lower potential than the N Region. The P Region has holes as majority carriers (consider them to be like positive charges) and the N Region has electrons as majority carriers.Between the P and N regions is the diode junction. This junction has a certain width called Junction Width. When the diode is reverse biased, the minority carriers (electrons on P side and holes on N side) move on to the junction layer and cause the junction to expand due to repulsion from like charges on the nearer plate. These minority carriers do not however cross onto the other side because of junction potential. If a sufficiently large reverse potential is applied, the junction keeps expanding till it eventually collapses and goes into Reverse Breakdown.

Answer 2

Answer: Any object that can store charge is a capacitor. Leakage or stray capacitance is a phenomenon that occurs when charge is accidentally stored. It can be found in transmission lines and in the insulation bushings of high-power devices like transformers, high-rating motors, etc.

Explanation:

• Simply separating the charges is all that is required to obtain capacitance. This often results from an electric field appearing across a dielectric substance. The charge can be seen on either side of the dielectric's surface. This is due to the insulating nature of dielectrics.
• In fact, an electric field will exist and charges will build up on the two conducting plates even if a field is applied across vacuum using two conducting plates at a distance. This is essentially what occurs in a diode.
• Take a look at a PN Junction diode. When the P Region is given a lower potential than the N Region, it is said to be reverse biased. Considered to be like positive charges, holes are the majority carriers in the P region, while electrons are the majority carriers in the N region.
• The diode junction is located between the P and N regions. This intersection's breadth is known as the Junction Width. The minority carriers (holes on the N side and electrons on the P side) travel to the junction layer when the diode is reverse biased, which causes the junction to grow as a result of repulsion from like charges on the nearby plate.
• However, due to the possibility of a junction, these minority carriers do not cross over. The junction will keep expanding if a strong enough reverse potential is given until it eventually collapses and enters Reverse Breakdown.

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