Final answer:
Decreasing the doping levels of a reverse-biased PN junction is the correct way to increase its capacitance, as it enlarges the depletion region, which is inversely related to capacitance.
Step-by-step explanation:
To increase the capacitance of a reverse-biased PN junction, one effective way is to decrease the doping levels. By reducing the number of impurities (dopants) added to the semiconductor material, the width of the depletion region becomes larger, which in turn increases capacitance. This is because the capacitance of a junction is inversely proportional to the width of the depletion region. In contrast, increasing the junction area does lead to an increased capacitance, but within the context of a reverse-biased PN junction, decreasing the doping is the most direct approach. Switching to a different material with a lower dielectric constant would not increase capacitance; in fact, it would do the opposite since the capacitance is directly proportional to the dielectric constant. Lastly, increasing the reverse bias voltage will generally increase the width of the depletion region, resulting in lower capacitance.