If you want to increase the reverse bias capacitance of a P+/N junction diode at a given reverse bias voltage, which of the following action (assume the diode remains to be P+/N diode after the change) is the most effective method?

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Question restatement: You want to increase the reverse bias capacitance Cj of a P+/N junction diode at a fixed reverse bias voltage, while keeping the device as a P+/N diode after the change. Answer options: 1) increase the P+ doping 2) reduce the P+ doping 3) increase the N doping 4) reduce the N doping.

Option 1: increase the P+ doping. In a P+/N junction, the P+ side is already very heavily doped. Increasing P+ doping further would have a diminishing return on reducing the depletion width because the depletion region tends to form where there is a balance of charges from both sides. The net effect on depletion width and thus on Cj is less dramatic than changing the N-doping, so this option is not the most effective.

Option 2: reduce the P+ doping. Lowering the P+ doping would typically widen the depletion region on the P side, increasing the overall depletion width and thereby decreasing the junction capacitance. This goes against the goal of increasing Cj, so this option is incorrect.

Option 3: increase the N doping. Increasing the donor concentration on the N side raises the charge density on that side, which reduces the depletion width W more effectively (since W scales inversely with the square root of the dopant concentration, among other factors). A narrower depletion region yields a larger reverse-bias capacitance Cj (Cj ≈ εA/W). This is the most direct and impactful way to boost Cj while keeping the device as a P+/N junction.

Option 4: reduce the N doping. Lowering the N-side dopant concentration would widen the depletion region, which decreases the capacitance. This directly opposes the goal, so this option is incorrect.

Summary reasoning: Capacitance in a reverse-biased PN junction is inversely related to the depletion width. Increasing dopant concentration on a given side reduces the depletion width and increases Cj. Among the choices, increasing the N-doping on the N side is the most effective way to shrink the depletion region and raise Cj, whereas decreasing dopant levels or altering the P+ side in the ways described would either have little effect or reduce capacitance.

EESM5200 (L1) Part I: Topic 8 - PN Junction Switching and Model

If you want to increase the reverse bias capacitance of a P+/N junction diode at a given reverse bias voltage, which of the following action (assume the diode remains to be P+/N diode after the change) is the most effective method?

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