LEDs can also be used as photodiodes because they can emit and sense light from their junction. All PN junctions are photosensitive and can be used in a photoconductive, undistorted voltage mode with the PN junction of the photodiode always reverse biased so that only the diodes can leak or dark current flow.
The working principle of a photodiode is that when a photon hits the diode with great energy, it forms an electron hole. This mechanism is also referred to as an internal photoelectric effect. When the absorption occurs in the barrier layer, the carriers are removed from the barrier layer by the built-in electric field of the barrier layer.
Therefore, holes in the region move toward the anode and electrons toward the cathode, and a photocurrent is generated. The total current through the diode is the sum of light failure and photocurrent. Therefore, the missing current must be reduced to maximize the sensitivity of the device.
