Can somebody tell me about photo transistor?
Answer:
In a normal transistor the base-emitter current times the beta of the transistor determines the current flow in the transistor.
In a photo transistor the base is designed to generate its own voltage when exposed to light. The exact voltage and therefore current in the base emitter circuit depends on the light level hitting the base. To find out the range of wavelengths that the photo transistor responds to refer to the data sheet.
A transistor is normally a 3 terminal device. You have a common node, an output node, and an input or control node. So a photo-transistor merely replaces the input or control node (normally voltage for an FET, and current for a BJT) with an optical input.
Transistors are used as switches (in the digital world) or as amplifiers (in the analog world).
A photodiode is a two junction device, where the leakage current is sensitive to light.
A photo transistor (NPN) is a 3 junction device, where the current mechanism is not leakage, but rather diffusion current.
They are not exactly the same, but they are quite similar.
I think the explanation at the following like explains the phototransistor much better than the wikipedia page and it helps to show the relationship between a photodiode and a phototransistor.
http://encyclobeamia.solarbotics.net/art...
The first point is that any BJT (bipolar junction transistor) would be a phototransistor if it were housed in a case with a transparent window. To understand the phototransistor one first needs to understand a regular NPN BJT. See
http://en.wikipedia.org/wiki/bipolar_jun...
Injecting “holes” into the base will permit electron (minority carriers) to defuse into the base region and since the base region is made very thin some of those electrons will reach the base collector junction before combining with holes. Any electrons that manage to make it to the base collector junction depletion region will be swept into the collector.
Light hitting the base region essentially has the same effect of injecting holes into the base. Photons will be absorbed by electrons in the base and knocked into the conduction band. Some of these electrons will defuse into the Base Collector depletion region and will get swept into the collector. The same process creates holes and some of these will defuse into the base emitter depletion region and be swept into the emitter. Once charge carriers exist in the reverse biased base collector region amplification will occur and electrons will start to flow from the emitter to the collector (across the base).
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