Photodiode

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 its connections. It is a diode and wired with reverse biasing so that when in the dark, only a small amount of current, known as dark current, will flow. When in the light, a current known as photocurrent flows through the diode. 
Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower as its surface area increases. The common, traditional solar cell used to generate electric solar power is a large area photodiode. Photodiodes and look the same physically but have different functions.
It is based on a reversed bias circuit.