Diodes are electronic components that let current flow in one direction along a wire but not the other. Photodiodes are specially constructed diodes that have an additional feature. When photodiodes are exposed to light, they generate their own electrical current. This feature allows photodiodes to function as light meters, solar cells, motion detectors and more.
Differences from Normal Diodes
All diodes are made out of two layers of specially treated silicon, n-type (treated with boron) and p-type (treated with phosphorous.) Photodiodes are also made of these two substances, but one of the layers is much thinner than with other diodes. This leaves photodiodes either mostly n-type with a thin p-type layer on top, or mostly p-type with a thin n-type layer on top. This thin layer is the light-sensitive layer and is often coated with a film that reduces reflection and enhances response to light. Photodiodes are also made of much purer silicon than normal diodes.
Operation
The thinness of the upper layer allows light to penetrate to a level where it is easily able to knock electrons out of their orbits in the silicon atoms. These electrons are pulled toward one side of the diode. This causes a current to flow through the diode and into the rest of the circuit that is proportional to the intensity of the light falling upon it.
Color Sensitivity
The thickness of the upper layer works together with the thickness and type of film coating it to make photodiodes more sensitive to certain colors of light than others. The specific light sensitivity of a photodiode is also determined by how much voltage is applied to it.
Proper Connection
Photodiodes must be given reverse bias to function properly. This reverse-biased voltage determines where the light sensitive region is within the photodiode. This means that the anode is connected to negative voltage and the cathode is connected to positive voltage, which is the reverse of how diodes are normally connected.
Uses
Photodiodes are used to make light meters, because the current they produce is proportional to the amount of light they receive. This makes them also useful as motion detectors. If an object moves in front of them then it interrupts the light it receives, changing the current. Photodiodes are also used in optocommunications, where data is sent by modulating lasers or other sources of light.
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