Transition And Diffusion Capacitance Of Pn Junction Diode Pdf

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With the increase of magnitude of reverse bias, majority carriers move away from the junction i. These uncovered immobile charge on the two sides of the junction constitute a capacitor of incremental capacitance C T given by,. This capacitance C T is called the transition capacitance or space charge capacitance or barrier capacitance or depletion layer capacitance.

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Transition and Diffusion capacitance in P-N junction diode

A p—n junction is a boundary or interface between two types of semiconductor materials , p-type and n-type , inside a single crystal of semiconductor. The "p" positive side contains an excess of holes , while the "n" negative side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping , for example by ion implantation , diffusion of dopants , or by epitaxy growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes. For example, a common type of transistor , the bipolar junction transistor , consists of two p—n junctions in series, in the form n—p—n or p—n—p; while a diode can be made from a single p-n junction.

In a p-n junction diode , two types of capacitance take place. They are,. We know that capacitors store electric charge in the form of electric field. The conducting plates or electrodes of the capacitor are good conductors of electricity. Therefore, they easily allow electric current through them. On the other hand, dielectric material or medium is poor conductor of electricity. Therefore, it does not allow electric current through it.

The transition capacitance represents the change in charge stored in the depletion region with respect to a change in junction voltage. The increase in the level of reverse bias caused the width of the depletion region, W to increase. An increase in the width of the depletion region, W is accompanied by additional uncovered ions in the space charge or transition region. Because positive ions exist on one side of the junction and negative ions on the other, the transition capacitance, CT is analogous to a parallel plate capacitor for which we have. For a step graded junction, the width of the depletion region, W is inversely proportional to the square root of the reverse bias voltage.

Capacitance of P-N Junction Diode Notes for Electronics Engineering 1st Year

We are developing the next version of CrazyEngineers. If you wish to receive latest updates and early access, click the link below. When P-N junction is reverse biased the depletion region act as an insulator or as a dielectric medium and the p-type an N-type region have low resistance and act as the plates. This junction capacitance is called as space charge capacitance or transition capacitance and is denoted as CT. Since reverse bias causes the majority charge carriers to move away from the junction , so the thickness of the depletion region denoted as W increases with the increase in reverse bias voltage.

If we pretend for a moment that we are hard-core electrical systems engineers, we do not care at all about what a pn -junction diode consists off, how it works, or how it is made. We simply describe it as a black box and use the two equations from before , but now with a possible frequency dependence thrown in for the output current. For small signal behavior we have by definition. The relation between U 0 and I 0 is simply given by the DC current voltage characteristics of our black box. In principle, the output signal amplitude i. All we have to do then is to tabulate or represent graphically the functions I m w , I 0 and j w , I 0 for our black box. This is all.

Capacitance of P-N Junction Diode Notes for Electronics Engineering 1st Year

Referring to Fig. What is the ratio of the change in capacitance to the change in voltage? Repeat part a for reverse-bias potentials of V and -1 V. Determine the ratio of the change in capacitance to the change in voltage.

Transition Capacitance

Diffusion capacitance

P-N junction diode is the most fundamental and the simplest electronics device. When one side of an intrinsic semiconductor is doped with acceptor i. Any variation of the charge within a p-n diode with an applied voltage variation yields a capacitance which must be added to the circuit model of a p-n diode. The capacitance associated with the charge variation in the depletion layer is called the junction capacitance, while the capacitance associated with the excess carriers in the quasi-neutral region is called the diffusion capacitance. Expressions for the capacitances are obtained by calculating the change in charge for a change in applied voltage, or:.

Since a junction forms between a P type and N type material it is called as P-N junction. One terminal of p-n junction diode is Anode and other is cathode. If the applied potential difference is more than the potential barrier, some holes and free electrons enter the depletion region. From the foregoing discussion the i-v characteristics of a p-n junction diode can be drawn as shown in Fig 2. The distance from one side of the barrier to the other side is called the width of the barrier, which depends upon the nature of the material. A PN junction is the simplest form of the semiconductor diode. A p-n junction is the metallurgical boundary between the n and p-regions of a semiconductor crystal.

Diffusion Capacitance is the capacitance due to transport of charge carriers between two terminals of a device, for example, the diffusion of carriers from anode to cathode in forward bias mode of a diode or from emitter to baseforward-biased junction for a transistor. In a semiconductor device with a current flowing through it for example, an ongoing transport of charge by diffusion at a particular moment there is necessarily some charge in the process of transit through the device. If the applied voltage changes to a different value and the current changes to a different value, a different amount of charge will be in transit in the new circumstances. The change in the amount of transiting charge divided by the change in the voltage causing it is the diffusion capacitance. The adjective "diffusion" is used because the original use of this term was for junction diodes, where the charge transport was via the diffusion mechanism. See Fick's laws of diffusion. That problem is a subject of continuing research under the topic of non-quasistatic effects.


The depletion region of the PN junction diode has high resistance. Hence, the depletion region acts like the dielectric or insulating material. Thus, PN junction can.


working of pn junction diode pdf

Transition Capacitance CT of Diode (Space Charge Capacitance)

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Transition Capacitance and Diffusion Capacitance of Diode

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