Photon counting for the masses

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The absolute maximum rating for reverse bias for these parts is 5V. Could you have damaged the LED by driving it to 24V reverse bias? Could this damage account for the behavior you see, or do you see the same thing with LEDs right off the bat?

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Yup, right away... I've got 100k ohm in series with the diode and monitor the voltage across the resistor. (so not that much power to hurt the LED). I was counting "room light" photons all day Friday. Count was fairly stable at 4.5 kHz, for several hours. I tried different LEDs from the same batch.. same thing (slightly different Vr)

I don't 'get' the 5 volt number. I tried some other LEDs and couldn't get them to break down at 50 volts! (that was the limit of the PS I had on hand.)

George H.

Yup, right away... I've got 100k ohm in series with the diode and monitor the voltage across the resistor. (so not that much power to hurt the LED). I was counting "room light" photons all day Friday. Count was fairly stable at 4.5 kHz, for several hours. I tried different LEDs from the same batch.. same thing (slightly different Vr)

I don't 'get' the 5 volt number. I tried some other LEDs and couldn't get them to break down at 50 volts! (that was the limit of the PS I had on hand.)

George H. ________________________________-

Do you have access to a curve tracer? If not, you can send me a few and I'll put them on my 577.

TM

I'm not sure you're counting single photons, especially if you're using room light. It could be some other effect.

I can't reproduce your photon findings with a red LED I have here. I detect no reverse current through the LED at 50V reverse bias with the scope set on 20mV/div. There is some noise present, but nothing like you describe.

What sweep rate and vertical sensitivity are you using? Digital or analog scope?

John S

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Hi John, Digital scope, here are some =91scope shots.

This first is with 1 second persistence

Here=92s a shot of one pulse

And the same thing with 1 sec persistence again

I think you need a GaP LED to see this. I saw nothing on the other LEDs that I tried. Send me an address and I=92ll stick a few in the mail.

George H.

I was just reading that GaP is an indirect semiconductor, and it's doped with something to use it as an LED.

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Thanks TM, no curve tracer here. But I can do an I-V 'by hand'. Does a curve tracer tell me anything else?

George H.

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Seems weird to me too! The count rate is linear with the intensity. (as well as I can measure it.) So what else could it be?

Want a few? (It's best to see things for yourself) I do wish someone else would dig up a GaP LED and try.

George H.

.highlandtechnology.com=A0 jlarkin at highlandtechnology dot com

Ok. My LEDs just are not worthy. But, I'll leave the research to you. Thanks for the offer.

Still, interesting to note that 50V isn't enough to cause reverse current. Thanks for that tidbit.

John S

Well, okay, I relent. I'll try to verify your findings.

Send to:

John Smith

On a sunny day (Mon, 30 Apr 2012 11:07:11 -0700 (PDT)) it happened George Herold wrote in :

Seems like those diodes that have 'geiger' mode, those breakdown when biased just below some voltage when hit by radiation. Does it also do that with a Thorium or other radioactive source as trigger? And those dides are very expensive, so hold on to those LEDs :-) What is the null (dark) affect, background radiation + cosmic pulses?

Why do you have negative-going spikes? What does your schematic look like?

John S

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Oh, well I was sending them into a little amp I have... to go on into a counter (50 ohm Z in). And the amp is inverting, the counter wants postive going pulses... the supply is floating, and I just have polarities switched around to make it come out the 'right' way in the end.

George H.

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Hi Jan, I don't have any radioactive sources lying around.

In the dark I see no pulses. (Room lights off a little black foam thing over the LED)

Expensive? I think they cost us $0.07 each.

George H.

Hey I found this paper which at least talks about optical gain in GaP LEDs.

Maybe I should search for "Geiger mode"

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OK some are in the mail. (The one with the 100k ohm resistor attached made the pics I posted... guaranteed to work.)

George

Thanks. I'll try to duplicate your findings.

John S

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The data sheet says it's a 700nm GaP device. Somewhere one should be able to find more technical data on GaP red LEDs: a cross section, what substrate it's grown on, lattice mismatch if any, and a bunch more stuff

Thanks TM, no curve tracer here. But I can do an I-V 'by hand'. Does a curve tracer tell me anything else?

George H.

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For a diode, no. Just easier to control the voltage and current. That plus I can go to 1600 volts.

Also, it will show all four quadrants in one display.

FWIW.

TM

Eyes are said to be able to detect single photons. It's enormously egalitarian that most of the masses have 2 of them. :)