Q.Compare a photodiode and a solar cell in terms of how each is biased and the purpose each device serves.

Dark current is the residual reverse current of a photodiode that exists purely due to thermal generation of carriers, independent of illumination.

A photodiode is a p-n junction diode operated in reverse bias, in which incident light (photons) generates electron-hole pairs that add to the reverse saturation current, so the reverse current increases with light intensity.

…

The V-I curve shows current increasing with reverse bias (third quadrant operation), characteristic of a photodiode operating in photoconductive mode where light generates carriers that flow under reverse bias.

A photodiode is deliberately reverse biased because in this mode the diode's own dark current is tiny and stable, so the extra current generated by absorbed photons (electron-hole pairs) stands out clearly and varies almost linearly with light intensity.

A photodiode is a p-n junction diode designed to be sensitive to light, used to detect optical signals and convert them into an electrical current. It is operated in reverse bias:

• In reverse bias, the junction's depletion region widens and normally only a very small reverse saturation current flows (due to thermally generated minority carriers).
• When light (photons with energy greater than the bandgap) falls on the junction, it generates additional electron-hole pairs. These carriers are swept across the junction by the reverse-bias electric field, adding to the reverse current. …

A photodiode is a p-n junction, operated in reverse bias, whose reverse current changes with the intensity of light falling on it — this makes it an optical signal detector.

A photodiode is fabricated with a transparent window so light can fall on or near the junction. It is operated under reverse bias. Normally the reverse saturation current is very small, but when photons of energy greater than the band gap strike the junction, they generate electron-hole pairs, which …

A reverse-biased photodiode's current responds linearly and quickly to incident light intensity, making it ideal for measuring light intensity.

A photodiode is a p-n junction diode designed to be operated in reverse bias and exposed to light through a transparent window. Photons incident on (and near) the junction generate electron-hole pairs; these are swept across the junction by the reverse-bias electric field, adding to the (otherwise very small) reverse saturation current. The magnitude of this reverse photocurrent increases almost linearly with the intensity of incident light, and the device responds quickly (fast rise/fall time), making a reverse-biased photodiode well suited as a sensitive, fast light-intensity (illumination) sensor/detector.

Why the other options are wrong: …