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Photo transistor

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New Member
hi all,
Please tell me what is a phototransistor?
How does it work?
Some handy info on them?
Lalit :)


Active Member
This semiconductor have a small lens (focused to base-emitter).The collector current depend from light intensity.(if base pin existiert, in dark You can use it as normal transistor...)Many years ago, i've made phototransistor from normal germanium transistor, who have black painted glass package.Without black cover work as light detector.


New Member

Sebi, i'm looking for a phototransistor with the code BPX 25. It's quite old (1995). I need this component to complete my project, but i just can't get it in my place(Malaysia). Can you teach me how to make a phototansistor by using the normal transistor, just like the way you said. Or, do you know any other phototransistor can replace this? Thanks


New Member

If you know about diodes then this might explain how a photodiode works, but it might also confuse you. Sorry if it does, its been a year since I did them at uni. I don’t mean to patronise you with the jargon either.

In a photodiode a PN junction is operated in reverse-bias voltage. Incident photons create excess electrons and holes in the space-charge region (which is the region where the p-type material and n-type material meet). These excess carriers are swept out of the space-charge region by the electric field creating current.

I don’t know about phototransistors but I would think that they would work on the same principal except based on a PNP or NPN transistor instead of a PN diode.
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