Explain The Origin Of Diffusion And Junction Ca

CLEANED Explain the origin of the diffusion and junction capacitance

CLEANED: Explain the origin of the diffusion and junction capacitance

The problem requires an explanation of the physical origins of diffusion and junction capacitance in p-n junction diodes, along with the conditions under which each type of the capacitance dominates. It is essential to understand that the total capacitance of a p-n junction is primarily due to two components: junction (depletion-layer) capacitance and diffusion (storage) capacitance. The dominance of either depends on the biasing condition and the corresponding charge distribution within the diode.

Junction (Depletion-Layer) Capacitance: This capacitance arises from the depletion region that forms at the p-n interface due to the diffusion of carriers. When the diode is reverse biased or lightly forward biased, the depletion layer widens, and its charge acts as a dielectric between the p-type and n-type regions. The depletion region behaves much like a parallel-plate capacitor, with the depletion width (W) playing the role of the separation between plates. The capacitance per unit area (Cj) can be expressed as:

Cj = εs / W

where εs is the permittivity of the semiconductor. This capacitance is highly dependent on the depletion width, which, in turn, depends on the applied bias voltage. Under reverse bias, the depletion region widens, increasing W and decreasing Cj. Conversely, under forward bias, the depletion narrows, decreasing W, and thus increasing Cj. This capacitance is called the geometrical or depletion-layer capacitance, as it is directly related to the physical extent of the depletion region.

Diffusion (Storage) Capacitance: This capacitance is associated with the excess minority carriers stored in the quasi-neutral regions adjacent to the depletion zone when the diode is forward biased. When a forward bias is applied, majority carriers in each region diffuse into the other side, creating a distribution of excess minority carriers. These carriers can be stored temporarily, creating a charge that contributes to the diode's overall capacitance. The diffusion capacitance becomes significant at large forward biases; it reflects the stored minority charge and depends on the diode's minority carrier lifetime and injection level.

In terms of dominance, the diffusion capacitance is prevalent under high forward bias conditions, where significant injection of minority carriers leads to an increased stored charge. The depletion capacitance dominates under small signals—particularly near the equilibrium or reverse bias—when the depletion region width is substantial and the stored minority carriers are minimal.

For illustrative purposes, schematics illustrating the depletion region as a capacitor, with a capacitor symbol across the depletion zone, can be used to depict the junction (depletion-layer) capacitance. Conversely, a schematic showing excess minority carriers stored in the quasi-neutral regions, contributing to an additional charge, illustrates the diffusion (storage) capacitance.

In summary, the physical origin of junction (depletion-layer) capacitance stems from the electric field across the depletion region acting as a dielectric, forming a capacitor whose value is controlled by the depletion width. The diffusion capacitance originates from the excess minority carriers stored in the quasi-neutral regions during forward bias, effectively introducing a charge storage component that varies with bias and injection levels. The dominance of each depends on the biasing condition, the magnitude of applied voltage, and the carrier dynamics within the diode.

References

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