A reverse bias is applied to a PN junction and a light is shined onto it. Assume the energy of the light is close to the bandgap of the semiconductor material. Which of the following change is likely to give to largest increase the magnitude of the reverse current of the PN junction?

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BlackTom AI · Accepted Answer

This question asks which change would most strongly increase the reverse current of a PN junction when illuminated with light near the bandgap.
Option 1: Increase the wavelength of the incident light. As the wavelength increases, the photon energy E = hv decreases. If the photon energy falls closer to or below the bandgap, fewer photons have enough energy to generate electron-hole pairs, so the generation rate and thus the photocurrent in reverse bias would tend to decrease. In short, longer wavelengths are less effective at producing a large photocurrent when near the bandgap.
Option 2: Decrease the wavelength of the incident light. Shorter wavelengths correspond to higher photon energy. If the photon energy remains above the bandgap, more photons have sufficient energy to create electron-hole pairs, increasing the generation rate of carriers. With illumination, more carriers are swept by the reverse field, contributing to a larger photocurrent. This option directly enhances the fundamental mechanism driving photocurrent under reverse bias in a near-bandgap regime.
Option 3: Increase the magnitude of the reverse bias voltage. While increasing reverse bias generally widens the depletion region and can improve carrier collection, the total photocurrent is largely determined by the generation rate set by the light. Beyond a certain point, the change in photocurrent with reverse bias is modest, and the question asks for the largest possible increase. Hence this option may raise collection efficiency but typically does not produce the largest increase compared with changing the illumination energy.
Option 4: Decrease the magnitude of the reverse bias voltage. Reducing reverse bias narrows the depletion region and can reduce the electric field that aids carrier separation and collection. This tends to decrease, not increase, the photocurrent under illumination, especially when the light energy is already near the bandgap. Therefore this option is unlikely to yield a larger reverse current.
In summary, options exploring photon energy mainly determine the rate at which electron-hole pairs are generated; among them, decreasing wavelength (increasing photon energy) typically yields a larger generation rate, leading to a larger reverse photocurrent compared with the other changes described. The other options either reduce the generation efficiency (longer wavelengths or weaker fields) or do not inherently provide a larger gain in the reverse-current magnitude under near-bandgap illumination.

A reverse bias is applied to a PN junction and a light is shined onto it. Assume the energy of the light is close to the bandgap of the semiconductor material. Which of the following change is likely to give to largest increase the magnitude of the reverse current of the PN junction?Single choice

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