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.
Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as its surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode. Photodiodes and phototransistors look the same physically but have different functions.
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