Silicon photodiodes. A photodiode is a semiconductor diode whose junction is exposed to light. When a photon is absorbed in the junction region, an electron-hole pair is created and a photocurrent is produced. The value of the photocurrent is proportional to the number of photons absorbed. Thus, measuring the photocurrent produced in a photodiode can be used to determine light intensity. Shown here are isolated photodiodes manufactured by EGandG Optoelectronics (model FNG-100). The outside diameter of the TO-5 package shown is 9.1 mm. Dec 2007.