Capacitance of p-n Junction
A capacitor has two parallel conducting plates (electrodes) separated by a dielectric
(insulator). The insulator stores electric charge in the form of electric field. The electrodes
of the capacitor allow electric current to flow in to them but the dielectric does not allow
electric current to flow through it, though it efficiently allows electric field. The capacitance
is directly proportional to the area of electrodes and inversely proportional to the distance
between them.
Similarly a reverse biased p-n junction diode stores electric charge in depletion region. The depletion region has immobile +ve and –ve carriers. In a reverse biased p-n junction, the p-type and n-type regions have low resistance and these act like the electrodes of the capacitor. The depletion region of the p-n junction diode has high resistance and it acts like the insulator. Thus, the p-n junction can be considered as a parallel plate capacitor. In depletion region, the electric charges (+ve and –ve ions) do not move. But, they exert electric field or electric force. Therefore, charge is stored in the depletion region in the form of electric field. The ability of a material to store electric charge is called capacitance. Thus, there exists a capacitance at the depletion region.
The capacitance at the depletion region changes with the change in applied voltage. When reverse bias is increased, a large number of holes (majority carriers) from p-side and electrons (majority carriers) from n-side are moved away from the p-n junction. As a result, the width of depletion region increases whereas the size of p-type and n-type regions (plates) decreases. We know that capacitance means the ability to store electric charge. The p-n junction diode with narrow depletion width and large p-type and n-type regions will store large amount of electric charge whereas the p-n junction diode with wide depletion width and small p-type and n-type regions will store only a small amount of electric charge. Therefore, the capacitance of the reverse biased p-n junction diode decreases when the applied voltage is increased.
Similarly a reverse biased p-n junction diode stores electric charge in depletion region. The depletion region has immobile +ve and –ve carriers. In a reverse biased p-n junction, the p-type and n-type regions have low resistance and these act like the electrodes of the capacitor. The depletion region of the p-n junction diode has high resistance and it acts like the insulator. Thus, the p-n junction can be considered as a parallel plate capacitor. In depletion region, the electric charges (+ve and –ve ions) do not move. But, they exert electric field or electric force. Therefore, charge is stored in the depletion region in the form of electric field. The ability of a material to store electric charge is called capacitance. Thus, there exists a capacitance at the depletion region.
The capacitance at the depletion region changes with the change in applied voltage. When reverse bias is increased, a large number of holes (majority carriers) from p-side and electrons (majority carriers) from n-side are moved away from the p-n junction. As a result, the width of depletion region increases whereas the size of p-type and n-type regions (plates) decreases. We know that capacitance means the ability to store electric charge. The p-n junction diode with narrow depletion width and large p-type and n-type regions will store large amount of electric charge whereas the p-n junction diode with wide depletion width and small p-type and n-type regions will store only a small amount of electric charge. Therefore, the capacitance of the reverse biased p-n junction diode decreases when the applied voltage is increased.
When forward bias voltage is applied to a p-n junction, the electrons (majority carriers) in
the n-region will move into the p-region and recombine with the holes. In a similar way,
the holes in the p-region will move into the n-region and recombine with electrons. As a
result, the width of depletion region decreases. A forward biased diode has very small
transition capacitance CT (which is neglected) and a relatively large diffusion capacitance
CD. The transition capacitance CT is defined as the change in electric charge caused by
an increase dV in the voltage applied across forward biased junction. CT = dQ/dV.
The electrons (majority carriers) which cross the depletion region and enter the p-region become minority carriers of the p-region. Similarly the holes (majority carriers) which cross the depletion region and enter the n-region become minority carriers of the n-region. A large number of charge carriers, which enter the other region will be accumulated near the boundary of the depletion region before they recombine with the majority carriers. As a result, a large amount of charge is stored at both sides of the depletion region.
The accumulation of holes in the n-region and electrons in the p-region is separated by a thin depletion region or depletion layer. This depletion region acts like a dielectric or an insulator of the capacitor and the charges stored at both sides of the depletion layer act like conducting plates of the capacitor, which is the origin of Diffusion capacitance.
If large current flows through the diode, a large amount of charge gets accumulated near the depletion layer, producing a large diffusion capacitance. Thus CD is proportional to the electric current or the applied voltage. When the width of depletion region decreases, the diffusion capacitance increases. The diffusion capacitance is: CD = dQ/dV, where dQ is the change in number of minority carriers stored outside the depletion region and dV is change in voltage applied across diode. The diffusion capacitance CD of a forward biased p-n junction is also called as storage capacitance. It occurs due to stored charge of minority carriers near the depletion region.
When forward bias voltage is applied to a p-n junction, the electrons (majority carriers) in the n-region will move into the p-region and recombine with the holes. In a similar way, the holes in the p-region will move into the n-region and recombine with electrons. As a result, the width of depletion region decreases.
The electrons (majority carriers) which cross the depletion region and enter the p-region become minority carriers of the p-region. Similarly the holes (majority carriers) which cross the depletion region and enter the n-region become minority carriers of the n-region. A large number of charge carriers, which enter the other region will be accumulated near the boundary of the depletion region before they recombine with the majority carriers. As a result, a large amount of charge is stored at both sides of the depletion region.
The accumulation of holes in the n-region and electrons in the p-region is separated by a thin depletion region or depletion layer. This depletion region acts like a dielectric or an insulator of the capacitor and the charges stored at both sides of the depletion layer act like conducting plates of the capacitor, which is the origin of Diffusion capacitance.
If large current flows through the diode, a large amount of charge gets accumulated near the depletion layer, producing a large diffusion capacitance. Thus CD is proportional to the electric current or the applied voltage. When the width of depletion region decreases, the diffusion capacitance increases. The diffusion capacitance is: CD = dQ/dV, where dQ is the change in number of minority carriers stored outside the depletion region and dV is change in voltage applied across diode. The diffusion capacitance CD of a forward biased p-n junction is also called as storage capacitance. It occurs due to stored charge of minority carriers near the depletion region.
When forward bias voltage is applied to a p-n junction, the electrons (majority carriers) in the n-region will move into the p-region and recombine with the holes. In a similar way, the holes in the p-region will move into the n-region and recombine with electrons. As a result, the width of depletion region decreases.
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