A photodiode is a light sensitive semiconductor. It is a type of photodetector capable of changing (or converting) light into either current or voltage, depending upon the mode of operation. If the anode and cathode leads of a photodiode are joined together by a wire, when in the dark, no current will flow. On the other hand, when in the light, current flows from the cathode to the anode. The diode part of a photodiode allows current to flow in only one direction.