Photodiodes are standard semiconductor devices used to detect the presence of light or photons and have the ability to generate current when these photons enter its semiconductor material. When photons enter the photodiode, a current is generated by the absorption of the photons.

It is important to distinguish the difference between a photodiode and a regular diode, specifically the differences in the junction of the doped materials.  Traditional diodes are created by a p-n junction, where p+ doped material is in contact with an n- doped material.  In a photodiode, there is not a traditional p-n junction, but rather a p-i-n junction where the “I” stands for an intrinsic material placed between the two p and n doped materials. This intrinsic region is where charge carriers accumulate from the adjacent p and n regions.  Once the intrinsic region is filled with sufficient charge carries, current can begin to flow.

Photodiodes are similar to traditional diodes in their voltage and current properties, except that they are designed to expose the semiconductor material to light or other UV sources to allow for photon absorption so that a photoelectric effect can occur and thus make the photodiode generate the current when the photons are absorbed.