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A lifetime ago, May 1974, a visiting professor to the EE program explained an odd 'side effect' in some transistors.

Photons are emitted when the electrons and holes recombine in the collector/base. And the photons could excite voltage production via PV effect in the base emitter junction.

He demonstrated the effect.

Normally this is a laboratory curiosity and we were told we'd never see any indication in the real world.
I wrote it down in my class notes, underlined and promised to look into the effect...and promptly almost forgot about it. Life was too filled with class work, then marriage, jobs, and hobbies...

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Today a friend sent me a link...

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The video is interesting and appears to demonstrate the effect.

Note: My 1980s 2N2222 barely worked, modern 2N2222 and modern 2N3904 worked extremely well. I intend to check every family of transistors I have.

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I can't think of any practical application, but the effect is interesting.

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Now to borrow a nightvision (starlight) scope and see if glass Ge and Si diodes emit any detectable light when forward biased.....

And somewhere I have some phototransistors, it will be interesting to see if they emit any detectable light when forward biased.

And will photovoltaic cells emit any detectable light when forward biased.

So many experiments, so little time....