Silicon Photodiodes:

The silicon diode housed in a transparent case and built in such a manner that visible light can strike the barrier among the p-type and n-type materials forms a photodiode. This is necessarily the reverse of an LED or IRED. The voltage is applied to the device in the opposite direction; therefore it ordinarily does not conduct current. Whenever visible light, IR, or ultraviolet (UV) rays strike the p-n junction, the current flows. The current is proportional to the intensity of the energy, in certain limits. The Silicon photodiodes are more sensitive at some wavelengths than at others.

Whenever energy of varying intensity strikes the p-n junction of a reverse biased silicon photodiode, the result current follows the fluctuations. This makes silicon photodiodes helpful for receiving modulated-light signals of the type used in fiber-optic and free-space laser communication systems. This effect reduces as the frequency increases. At very-very high frequencies, the diode performs like a capacitor since of its relatively high junction capacitance, and the effectiveness of the device as a modulated-light sensor is tainted.