Dark current is the unwanted leakage current of a pn junction photodiode in the reverse direction, when it is exposed to light. Such a diode is designed to generate a current which is proportional to the light incident on it.
In physics and electrical engineering, dark current is the relatively small electrical current that flows through photosensitive devices such as a photodiode or a charge coupled device, even when no photons enter the device. It consists of the charges generated in the detector when no external radiation enters the detector. In non-optical devices, it is referred to as a latch leakage current and is present in all diodes. Physically, dark current is due to the accidental generation of electrons and holes in the depletion region of the device.
The dark current serves as a “basic current” that can not be avoided. To achieve maximum sensitivity, you want the on / off ratio of the current to be maximum. Therefore, you want to minimize the dark current and turn-off current and maximize the inrush current response for a particular light source. The desired “on” current results from the photon absorption and the formation of electron-hole pairs in the depletion region of the diode.