reverse breakdown voltage

Oh no didn't you lose credit in school for too many digits! :^)

I stuck a red LED backwards with a 100k ohm resistor is series... at about 24V I got all these 'beautiful' RC spikes. ~ 1 volt peak, rc time ~20us. Cranking up the voltage more and it was noise everywhere, on a DC background. (I was monitoring the voltage across the resistor.)

Any reason you need to know this?

George H.

I don't remember. I never gave too many digits when I was in school :). I thought about trimming it, but nah, not worth the trouble :).

RC? What C? RC spikes? 1V at the resistor and 23V at the diode doesn't look like a huge breakdown. Maybe I'll really connect a rectifier to

325V - -LED+ - 10MOhm (input resistance of multimeter) - 0V

Mostly curiosity. I have various LEDs here, including 3 Cree XP-G R5 cool white, which are specified as 5V reverse voltage.

Bernhard

Did you also subject the LED to various light sources concentrated on the package while doing this? Might be interesting on the results differences.

Jamie

On Wed, 25 Apr 2012 17:39:42 -0400, Jamie Bernhard

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Yeah I did, When you biased it right to the breakdown region you could definitely see the effect of sticking an light up to it. Try it. Everyone's got a red 5mm LED, 100k ohm and ~30V power supply.

George H.

The voltage across the 100k R looked like

Oh, I've already done this not to long ago when I was pondering on an idea.

How ever, I am trying to see where you are getting the noise from? When I did it, all I saw on the scope was a PV effect at 1 uA, it could've been more, been a while.

I used my over head light which is nothing more than a incandescent

But I was able to get over 1 V as a PV with out any biasing, just the LED on the leads. It would make for a nice photo detector in a pinch though if you have the hi-z circuit for it.

I guess if one was to have a few LED's in series with HV applied and wait for a few photons to stop by for visit it would then act as a photo multiplier and push things a long, thus giving you a very sensitive detector. Or at least I think it would :)

I'll have to set that experiment up again at work, this time using the Lecroy maybe and see if I can see anything I may have missed before.

Jamie

Hmm, the DMM showed the full input voltage. No voltage drop due to the LED. IIRC before that I tried the LED in forward direction and got some voltage drop. However, after trying the reverse direction the behaviour was the same in either direction. No light, except for a short flash when I closed the circuit. The DMM showed ca. 330 mV diode drop and 445 Ohm resistance, both in each direction. I was convinced the LED was dead, but when hooked up normally it was still shining without appearant loss in brightness.

Ohh, I also had a 10uF film capacitor across the input voltage.

Bernhard

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Hi Jamie, Here=92s some quick =91scope shots. (Still set up on my bench.)

The first is just when the PD starts to reverse conduct, with no light.

And then at same voltage with the light shinning.

The shot of a single pulse.

And then with a higher reverse voltage... conducting all the time.

(Voltage scale changed for the last.)

George H.

The currents he is seeing are in the range of possible photodiode operation.

Cool. I wonder if you're approaching some sort of single-photon avalanche/Geiger mode. The dark behavior looks like a typical small zener diode at low current, a really noisy sawtooth oscillation.

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Yeah the low current looks pretty much like a ~20V zener biased to the corner. Where I assume it's the whole diode capacitance discharging..

Here I've got 100k ohm and ~10-20 us TC so 100- 200pF. That seems like a lot for a little LED. .. ohh.. I just had some coax with 'grabber clips'... ~170pF...

A x10 probe tomorrow.

Hey for those digital type guys, you could stick a comparator on the output and count pulses. As you keep increasing the voltage you hit a quiet region, where it looks resistive.

Well I've only looked at one red LED. (I hope it's not a golden part.)

George H.

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HEY! I think I may be counting single photons! (At extremely low efficiency) This is amazing. (So just try it yourself.)

A variable power supply reverse biased red LED in 5mm package (Vr~24 volts). 100 kohm current limiting (and sense) resistor, look at current through the 100k ohm with a x10 scope probe. Adjust voltage till you see some RC spikes. Now cover the LED with something black. The pulses stop! I turned out the lights in my lab and can still 'see' the light that's coming in from the room next door. I need a bit of buffering so I can send the pulses into a counter.

The count rate is highly dependent on the bias level.

Total Friday Fun! (I've gotta put some final changes into a pcb too.)

I should hit up google 'cause it seems like someone must have done this before.

George H.

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Just a quick addition, At the higher voltages it's not as light sensitive.

The diode is just avalanching away on it's own... with or without any light. Geo

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You may be in possession of a Geiger mode photon detector at about

I wonder how you could demonstrate single photon counting. Probably put the detector in a box with an LED and a lot of optical attenuation, and drive the LED on/off, and measure something.

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That _is_ fun. Do you have enough ND filters lying around that you can make a 1 photon per millisecond per die area light source for it? You can figure out the threshold number of photons by looking at how the count rate goes up with light level-- a few ND1 filters in a row would allow a simple measurement of that.

Cheers

Phil Hobbs

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So I tried comparing light levels with a photodiode. In the room lights this was hopeless. (vastly different wavelength sensitivity.)

But then I turned off the room lights and stuck a red LED onto a PD and then onto the reversed biased (RB)LED. Trying for maximum signal each time.

The numbers for the RBLED are a bit scattered. The maiximum light input was 'tweaky' wrt angle and position. Here's some data

Vf red led photodiode RBLED

0 0.004 0-3 some light in the room.

Then the same but with a yellow (high brightness) LED as the light source.

Vf PD RBLED

So the RBLED is a lot more sensitive to the yellow light. (as you would expect.) The count rate is at least roughly linear with the light intensity. (I think that means it can't be some weird two photon process that gives the pulses.) At the highest count rates for the yellow LED I was getting some overlapped pulses on the scope... those might not have been counted. So the apparent fall off in sensitivity for the high light intensity yellow LED is expected. I've got a piece of coax with clip leads connecting it to a little amp and the pulse width is still about 10-20 us. I need a little one or two transistor buffer to hang on it.

The photon generated count rate is a strong function of the bias voltage. All the above data was taken with Vb=3D 24.3V. This gives counts in the room light of ~450 per sec. At 24.9 V the room light count rate is 4800. (And this is still all photon generated, I can trun off the lights and cover it with black plastic and get zero counts per second.

I'm not sure if I answered your threshold question or not Phil.

Well I tried another brand of red LED. (Avago HLMP-EG30-NR000)

The LEDs we purchased were from Newark part number 92N5331. No longer carried. :^(

So I don't know if these are 'special' in some way. I tried three of them from the drawer and all avalanched at ~24 volts.

I've got a few other different color LEDS that I'll try.

George H.