Photodiode

NCERT explains that a photodiode is a p-n junction operated under reverse bias and illuminated through a transparent window. Incident photons with energy greater than the band gap generate additional electron-hole pairs in and near the depletion region, and these light-generated minority carriers add to the reverse current. Because the number of such pairs increases with the intensity of the incident light, the reverse current varies measurably and nearly linearly with intensity, making the device an accurate light detector. Reverse bias is preferred because the change in reverse current with illumination is easier to measure than the corresponding change under forward bias. The option that forward current stays constant is wrong because it neither uses reverse operation nor the light dependence that makes detection possible. The option that reverse bias prevents carrier collection is wrong because the reverse field actually helps sweep the light-generated carriers across the junction. The option that light has no effect is wrong because photogeneration is the very basis of the device. As stated in NCERT Class 12, Chapter 14, photodiode reverse current tracks illumination. A consistency check confirms that greater intensity yields proportionally larger current, ideal for sensing.