PD, Photodiode Image

Photodiodes are P-N junctions specifically designed to optimize their inherent photosensitivity. Photodiodes can be used two ways -- in a photovoltaic (here it becomes a current source when illuminated -- see solar cell), or photoconductive role. To use a photodiode in its photoconductive mode, the photodiode is reverse-biased; the photodiode will then allow a current to flow when it is illuminated.

ThermoCentrovision has an interesting site on the technology behind photodiodes here. See also the diode discussion here.

Note that phototransistors behave much like photodiodes, but with higher gain (i.e., a phototransistor allows more current to flow than would a photodiode in a photoconductive role). You can use this diagram to help you see the difference (both circuits are equivalent):


For an illuminating comparison of the various photo-sensitive devices, make sure to check out "Choosing the Detector for your Unique Light Sensing Application."

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Page author: Eric Seale  
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