LED output vs bias current

George Herold · 2013-10-04T13:13:45+00:00

So I was shinning some Cree leds onto a photodiode and measuring the photoc urrent. Here?s a plot (opamp bias current ~20pA subtracted from data.) Below about 1mA of LED bias current the photo current increases as the 3/2 power of the led current. Which was totally unexpected! (I expected linea r.) I?m making up a hand-waving explanation that?s got a non-radiative rec ombination channel. And then observing that for radiatvie recombination, i t?s not only the number of charge carriers at the boundary.. but they als o 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.) Does that make sense? Is this known? Thanks George H.

Oct 04, 2013 20 Replies

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Yeah.. well 3/2 till something near 1mA at which point it becomes linear. (You can do a decent job measuring PD DC current with the uA input on a dec ent DMM.)

Here's a plot of count rate vs photodiode current (where I used the same LE D current for both measurments.)

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Almost linear. (slope should be 1.0 'in theory') There are several things that 'screw up' the spad counts. At low count rates there is after pulsin g which gives you more counts. And then at high count rates you need to ta ke the dead time of the spad into account. I've 'semi'* corrected the data for the dead time effects.

Thanks for the IR led offer, but if I can't buy ~100's of them I'm not real ly interested. But drop me an email and I'll put some AND113R's in an env elope. (gherold-at-teachspin.com)(That's what gave the above data... In wh at might be called an extreme case of 'trud' polishing I filed the plastic lens of the 'spad' down to just above the active element and got lot's more counts with green photons.)

George H.

*so to first order the dead time correction says that the 'real' time I was measuring is the (gate time)-(dead time)*(number counts). But then there' s a second order effect that a second pulse during the dead time strecthes the dead time by a factor between 1 and 2... I ignored the second order cor rection.