The most important question of
Junction Diode Characteristics in Electronic Devices and Circuits;
Explain Transition capacitance and Diffusion capacitances?, is being answered here.
TRANSITION AND DIFFUSION CAPACITANCE:
Electronic devices are inherently sensitive to very high frequencies. Most shunt capacitive effects that can be ignored at lower frequencies because the reactance X
C=1/2πfC is very large (open-circuit equivalent). This, however, cannot be ignored at very high frequencies. X
C will become sufficiently small due to the high value of f to introduce a low-reactance "shorting" path. In the p-n semiconductor diode, there are two capacitive effects to be considered. In the reversebias region we have the transition- or depletion-region capacitance (C
T), while in the forwardbias region we have the diffusion (C
D) or storage capacitance.Recall that the basic equation for the capacitance of a parallel-plate capacitor is defined by C = €A/d, where € is the permittivity of the dielectric (insulator) between the plates of area A separated by a distance d. In the reversebias region there is a depletion region (free of carriers) that behaves essentially like an insulator between the layers of opposite charge. Since the depletion width (d) will increase with increased reverse-bias potential, the resulting transition capacitance will decrease. The fact that the capacitance is dependent on the applied reverse-bias potential has application in a number of electronic systems. Although the effect described above will also be present in the forward-bias region, it is overshadowed by a capacitance effect directly dependent on the rate at which charge is injected into the regions just outside the depletion region. The capacitive effects described above are represented by a capacitor in parallel with the ideal diode, as shown in Fig. 1.38. For low- or mid-frequency applications (except in the power area), however, the capacitor is normally not included in the diode symbol.
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| Effect of Transition capacitance and Diffusion capacitances on the semiconductor diode |
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