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There is this long tail on the step response. Here?s the same picture with the timebase slowed down.
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Looks something like a 2-2.5 us tail on the step.
What?s it caused by? I first thought it was perhaps heating of the LED during the step. But I was reading ?Photodetectors? by S. Donati, over the weekend. In Chapter 5 he talks about two sets of photo- generated carriers the majority are made in the depletion layer, and travel with the drift speed, (times in the nano-second range.) The second smaller set are those generated in the doped region. If they are within a diffusion length of the depletion region then these have a chance of adding to the photocurrent. Donati says that these have a time constant in the microsecond range! I wonder if this is what I am seeing?
I looked at the step response with a different color LED. (you expect more absorption in the doped region with shorter wavelength light.) So here is the pulse response from a 635 nm red and 594 nm Amber LED.
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The red LED is the trace that is a bit higher. I?m looking to see if I can dig up any blue or white LED?s.
Any other ideas how I might determine if that's what I'm seeing...
Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed biased at ~12V.
George H.
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Hi George,
This same problem plagued me some 10 years ago and was solved here. Some of the regulars here helped me getting the proper ideas. Unfortunately the thread is not archived anymore (can't find it).
The problem was caused by photons falling OUTSIDE the ring that surrounds the active region. The electrons there drift slowly into the active region and then get measured. The same timing and tail shape was seen there.
The problem was solved by hacksawing my $100 PD and painting the rim black. Result: a perfect square response (I was using it to test the squareness of laser pulses). Later we redesigned the optics so the light only fell in the centre.
Happy hacksawing! Arie de Muynck