SILICON PHOTODIODES

A silicon diode housed in a transparent case and constructed in such a way that visible light can strike the barrier between the p-type and n-type materials forms a photodiode. This is essentially the opposite of an LED or IRED. A voltage is applied to the device in the reverse direction, so it ordinarily does not conduct current. When visible light, IR, or ultraviolet (UV) rays strike the p-n junction, current flows. The current is proportional to the intensity of the energy, within certain limits. Silicon photodiodes are more sensitive at some wavelengths than at others.
When energy of varying intensity strikes the p-n junction of a reversebiased silicon photodiode, the output current follows the fluctuations. This makes silicon photodiodes useful for receiving modulated-light signals of the kind used in fiberoptic and free-space laser communication systems. This effect diminishes as the frequency increases. At very high frequencies, the diode acts like a capacitor because of its relatively high junction capacitance, and the efficiency of the device as a modulated-light sensor is degraded.