LED output vs bias current

Nice plot!

The existence of the knee is well known, but I never really paid much attention to how the light output behaved down there. "Trap sites" and "surface states" are the solid state physicist's version of "profit taking" and "program trading". ;)

Cheers

Phil Hobbs

We've done our own optocouplers, an IR led and a photodiode. They were linear from mA to low currents, microamps of LED current as I recall. We have plots somewhere. The nonlinearity was in the LEDS.

otocurrent. Here?s a plot (opamp bias current ~20pA subtracted from data .)

recombination channel. And then observing that for radiatvie recombinatio n, it?s not only the number of charge carriers at the boundary.. but they also have to find a ?dancing partner? on the other side. So some kind of factor that has do with hole and electron wave function overlap. And for two dimensions that could go as the square root of the number density. (I think.)

You mean the knee at ~1mA where output becomes more linear with drive curre nt?

I've been trying to get a square root factor to come out mathematically, fr om some overlap factor. But that looks to be linear too.. So the plot woul d have gone as I^2. not I^3/2. I'm missing something.

George H.

tocurrent. Here?s a plot (opamp bias current ~20pA subtracted from data. )

/2 power of the led current. Which was totally unexpected! (I expected li near.)

recombination channel. And then observing that for radiatvie recombination , it?s not only the number of charge carriers at the boundary.. but they also have to find a ?dancing partner? on the other side. So some kind of factor that has do with hole and electron wave function overlap. And f or two dimensions that could go as the square root of the number density. (I think.)

near

ots

Hmm, Well for my 'silly model'* if there's not much probability of non-radi atvie recombo, then it should be linear starting at a lower current. I've got an IR led I can try.

George H.

If memory serves, triode vacuum tubes follow a 3/2 power law as well.

There may be a physical parallel of some kind at work.

Joe Gwinn

Hmm, is that the space charge 'thing'? (I'm mostly clueless about tubes. I could run the tube tester and figure out which tube to replace, but not much more.) To throw a monkey wrench into it, here's the IR led data.

It's much closer to 2 at low currents and then bends over.

Above 10mA I start to see warming effects of the led. The intensity droops with a few second time constant. I should do a pulsed measurement, but that too much work.

George H.

This is interesting. One would expect the photodiode and LED are just reversed processes. I have always assumed photodiodes to be linear at low light levels and that it has likely been checked with photon counting. Could the beam pattern of the LED be changing with current level??

Paul C.

Yes, photodiodes are amazingly linear, much better than LEDs.

Nah, LEDs are more complicated devices by a lot--double heterojunctions in nasty compound semiconductors, for a start.

The best photodiodes are good for 14 decades of linearity at room temperature, if you're willing to wait. See Eppeldauer et al.,

Cheers

Phil Hobbs

Good to know on the photodiodes. Been dinking around with LEDs as SPADs based on a thread here some time ago. Have a handfull of Siemens GaAs SFH431 that breakdown at

Paul C.

rom

other

Excellent! Hey try some AND113R's. I mistakenly said before that these do n't work, but I was wrong! (I get several bags of red led's in and one bag looks like another... anyway.) If you then file the top of the led down y ou can get some decent count rates with a green led as light source. Oh on e thing about spads is that the higher you bias them above the breakdown vo ltage the higher the efficiency... And the pulse height is basically equal to the bias voltage minus the breakdown voltage... so if you've only got 10 mV crank up the bias voltage. If that causes the pulses not to quench then make the quecnh resistor bigger.

RE: LED nonlinearity. So imagine that an led has two ways for the charge c arriers to recombine. one is e-h recombination the other some trap state o r other 'thing'. But this second gives no light. Now the number of traps is fixed. And so the number of e's that go through the trap depends only o n the number of e's. But to recombine with a hole (h) the electron and hol e have to find each other. So the number of e-h recombo's should go someth ing like the density squared. (I know I got a 3/2's power.. go figure.)

Does that make any sense?

George H.

Hey are those SFH431's still being made? (I can't find 'em.)

George H.

Back in the mid 90's I went through an electronics kick. At that time there was an electronic shop near me that would sell garbage bags full of out of spec ICs, resistors and caps. One of the bags contained LED's of varius types including 20 or so two lead SFH431s. I assume they are out of print when I bought them like all the rest of the stuff I'm still playing with.

There's a really good paper I found on the interweb

"Avalanche photodiodes and quenching circuits for single-photon detection" Applied Optics / Vol. 35, No 12 / 20 April 1996.

From what I see on the scope the SFH431 are typical of the "thin" junction SPADs of the day. I expect (but don't know) they have rather bad secondary pulses compared with a real SPAD which is why I'm looking into quenching circuits.

Paul C.

Make sense? I don't know. You might try running the same LED as photodiode, some of them work rather well. Use a real photodiode and an LED-as-photodiode to monitor the same light source. Would you still get the same non-linearity??

Paul C.

Next time, quote the author's names. Sergio Cova and his team at the Milan Polytechnic are magic, and their work has been recommended here before.

Do another experiment, block or divert 3/4 of the light from the led, does start of the linear section move.

Hey you read my mind. That is the intent of the LED. (to show that a the count rate of spad is linear with light intensity.) The spad has a few iss ues... it's not as good at low intensity, and second it poops out at higher count rates due to the built in dead time. I've got data.. I'll have to p lot it up.

On the 3/2 power law front. I got a message from someone on another forum that the 3/2 power law is known and the explanation is that the non-radiati ve trap recombo is *not* linear in the charge density. If one assumes a ra ndom (Gaussian) trap distribution then the number of recombo's through the traps goes as the 3/2 power of the density... Which then leaves a 3/2 power for the increase in the number of photons. (Though I'm hoping to find a p aper.)

George H.

otocurrent. Here?s a plot (opamp bias current ~20pA subtracted fro m data.)

n-radiative recombination channel. And then observing that for radiatvie recombination, it?s not only the number of charge carriers at the b oundary.. but they also have to find a ?dancing partner? on the other side. So some kind of factor that has do with hole and electron wave function overlap. And for two dimensions that could go as the squar e root of the number density. (I think.)

Oh, you want me to test the photodiode linearity?

George H.

Cool. So, in photon counting mode do you expect a 3/2 law?? BTW I would be happy to send you a few of the IR LEDs. My email is with earthlink.net