Question

What do you mean by Junction Capacitance

Answer 1

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Junction capacitance is the capacitance which forms in a PN junction diode under reverse bias. In a normal capacitor, the two parallel conducting plates are electrodes which allow the conduction. When a voltage or potential difference is applied across a capacitor the charges accumulates at the electrodes.

Answer 2

Explanation:

Junction Capacitance

Junction capacitance is the capacitance which forms in a PN junction diode under reverse bias. In a normal capacitor, the two parallel conducting plates are electrodes which allow the conduction. Whereas, the medium between two parallel conducting plates is purely insulating dielectric material which does not allow conduction.

When a voltage or potential difference is applied across a capacitor the charges accumulates at the electrodes. The voltage will not move through the dielectric. But the dielectric medium will allow an electric field to flow through it when huge number of charges accumulates at the electrodes. However, the electrodes are able to store electric charge. This ability of storing electric charge is known as capacitance.

Figure 1 is the normal parallel plate capacitors yielding capacitance.

Here,

Here, C is capacitance, dQ is change in charge, and dV is change in applied voltage.

In similar manner, when reverse bias voltage applied across a PN junction diode, the two regions P and N are behaves 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 ability of storing the charge is nothing but capacitance which in fact is termed in PN junction diode as junction capacitance or transition capacitance or depletion capacitance.

Figure 2 is the PN junction diode under reverse bias condition.

When a reverse bias voltage applied across a PN junction diode, the holes form p – type region (majority carriers) moves away from the junction and the electrons from the n – type region (majority carriers) moves away from the junction. Here, the width of depletion region will increase. However, the depletion region acts as a dielectric medium and two electrodes store the electric charge.

When the voltage applied increases, the depletion region will increase. However, the size of P and N region is decreased. Consequently, the ability to store electric charge will be reduced.

The PN junction diode with narrow depletion width and large P and N regions can store more electric charge. The diode with broad depletion width and small P and N regions can store less electric charge. Therefore, the applied voltage is inversely proportional to junction capacitance formed in a PN junction diode under reverse bias condition.

Here, Cj is junction capacitance, ε s is permittivity of semiconductor material, A is area of P and N type regions (electrodes) and w is width of the depletion region.

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