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PD Photocurrent rise/fall time an unexpectedly strong function of wavelength?!

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Kleinstein:
Those LDRs used in the photo choppers are a bit different from the ones to just measure brightness: they are faster and this comes with reduced sensitivity. The LDRs get good sensitity from long lived traps, that capture electrons or holes and this way conserve the extra photoconductivity for a longer time. Without that the photo conductive effect is rather low sensitivty.

The tricky part is to get some medium speed trapping centers to get enough sensitivity, but also only few long lived trapping centers that give a slow tail.  Quite often there are a few very slow parts and it takes really long (could be hours) to get back all the way to the dark resistivity.

RJSV:
Yes, Kleinstein, I'm trying to study / comprehend that reverse recovery concept, doing work on solar cells, in some other EEVBLOG posts.
   I think I'm seeing some hidiously long responses, like 1.2 mSec but also confusing is a little 4-pin IC doing comparator switched output. Going to take a little time to comprehend.

Kleinstein:
Relatively pure silicon usually has rather few deep traps, though one can not totally exclude them. In photo conduction of germanium I have seen a slow component  - at low temperature it got really slow, but at room temerature it was not so bad. Looking at the reverse recovery at different temperature sometimes runs under the fancy name of deep level transisent spectroscopy.

Besides the electronic effect there can also a thermal effects. 1.2 ms is about in the time scale for thermal equilibration for a normal thickness silicon wafer. When driven in forward direction the heat generation is not that uniform across the thickness of the solar cell - there are even regions (e.g. the contacts if there is not much recombination at the contracts) that see cooling from peltier effect. At university I did my diploma thesis about the heat production in this case. In addition there can be lateral concentration of the current - this would tend to be slower to get thermal equilibrium.

RJSV:
Glad to see that 1.2 mSec delayed photo- diode isn't out of bounds.  I've got Tektronics scope right here in the study room, and all the parts, (just busy), so I should be emphasizing doing all those needed checks.
Thanks, lots.

RJSV:
   (Some more info, on Solar Cell interface):
  The 1.2 mSec delay I quoted was an average, as both switching directions happen, via 'inverter' chain, in my implementation of a classical Edge Detect; that's with several 'gates' feeding in serial order, into a two input logical 'AND'.
   In some related trials, I did quite easily manage to simply connect one of those 'garden light' gates as a straight replacement to a regular push button, in one TOY keyboard.  That works to play a piano note, or to cause a canned song to play.
Interesting part of that is the solar light, as a 'gate' does work in EITHER connection config...That is, if we assume that toy does key scanning by pulling down a 'row', while monitoring a set of 8 columns in typical style.  One direction of connecting not super good, but reversed worked quite well.  I'm a bit concerned, about possibly supplying excess voltage, in that substitution, as solar cell unit, (don't forget, output is 222 khz), can put out actual voltage, vs a passive dome push-button, but seems to work, by way of small flashlight.
Kind of weird, that it works in both directions of hook-up.
  I need some more time, for more results, as I need to construct (the optical edge detect, using several gates)
that being on the test bench.  Plus, I need to get a periodic action, for best oscilloscope capture, of those optical rise and fall times.  That, to characterize for actual switching time, as evb149 has been describing, the large delay seen in photo-voltaic output, in one direction of switching.  Thanks.

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