The Working Principle Of Photodiodes

Photodiodes, as a key device that converts optical signals into electrical signals, play a crucial role in modern electronic technology. From communication, imaging to sensor technology, the application of photodiodes is ubiquitous, bringing revolutionary progress to modern technology.
The working principle of photodiodes is mainly based on the photoelectric effect and the PN junction principle. The photoelectric effect refers to the phenomenon where when light is irradiated onto certain substances, the electrons inside these substances absorb the energy of photons, resulting in transitions and the formation of electron hole pairs. In photodiodes, this photoelectric effect occurs in the PN junction region.
The PN junction is the core structure of a photodiode, consisting of a P-type semiconductor and an N-type semiconductor. At the PN junction, due to the diffusion of holes from the P-type semiconductor to the N-type semiconductor and electrons from the N-type semiconductor to the P-type semiconductor, a built-in electric field is formed. The direction of this built-in electric field points from N to P, preventing further diffusion of holes and electrons, thus forming a very thin depletion layer at the PN junction interface.
When light is irradiated onto the PN junction of a photodiode, photons are absorbed by atoms or molecules in the depletion layer, causing electrons to transition from the valence band to the conduction band, producing electron hole pairs. Under the action of the built-in electric field, electrons move towards the N region and holes move towards the P region, thereby forming a photo induced electromotive force on both sides of the PN junction. The magnitude of this photo induced electromotive force is directly proportional to the intensity of the incident light.
During the operation of a photodiode, it is usually necessary to apply a reverse bias voltage to it. This is because the reverse bias voltage can further enhance the effect of the built-in electric field, making the photo generated electromotive force more pronounced. At the same time, reverse bias voltage can also suppress dark current (i.e. weak current generated when there is no light), improve the sensitivity and signal-to-noise ratio of photodiodes.