The photodiode is a special diode that converts the light signal into a current or voltage signal. Its structure is basically the same as that of a traditional diode. It has a PN junction. However, when designing and manufacturing the photodiode, try to make the area of the PN junction smaller. Large, in order to receive incident light.

　　 Its basic principle is: when light hits the photodiode, the absorbed light energy is converted into electrical energy. The photodiode works under reverse voltage, and only passes a very weak current (generally less than 0.1 microampere), called dark current. When there is light, the photon with energy enters the PN junction and transfers the energy to the covalent bond. Electrons cause some electrons to break away from the covalent bond and generate electron-hole pairs, which are called photo-generated carriers, because the number of photo-generated carriers is limited, and the number of multi-carriers before light is much greater than the number of photo-generated carriers. Therefore, the photo-generated carrier has a small effect on the multi-carrier, but the number of the minority carrier is small, which has a greater impact. This is why the photodiode works under a reverse voltage instead of a forward voltage.

Under the reverse voltage of photogenerated electrons, under the action of photogenerated carriers, in order to encourage minority carriers to participate in the drifting motion, in the P zone, the photogenerated electrons diffuse in the PN zone. If the thickness of the P zone is less than the electron diffusion length, the photogenerated electrons Will be able to pass through the P area to reach the PN junction.

The work of the photodiode is an absorption process, which converts the change of light into the change of the reverse current. The combination of photocurrent and dark current is photocurrent, so the dark current of the photodiode minimizes the sensitivity of the device to light. The intensity of light is proportional to the photocurrent, so that the light signal can be converted into an electrical signal.