Hi, folks:-
I am looking at using an array of BPW34S photodiodes to detect a visible red laser.
Reverse biased by about 0.5V.
The sensitive area is 3mm x 3mm.
What happens to the photocurrent if only part of the PD sensitive area is illuminated?
--Spehro Pefhany
red laser.
illuminated?
What ought to happen is that illuminated bits will photoconduct and the unl it areas won't.
If you illuminate a reverse-biased diode, individual photons generate indiv idual charge-carrier pairs in the reverse biased layer, and the pairs separ ate and the two elementary charges involved head off to their preferred ele ctrode and constitute a charge transfer between the two electrodes, also kn own as a photocurrent. It looks exactly the same as dark current, but - if you've got enough incident light - there will be more of it.
I suppose if you had very intense and very localised photo-illumination you might have a problem with local space charge in the reverse-biassed layer, and some of the charge carriers might take a bit longer to get to the elec trodes, but only Phil Hobbs would know about that.
You're more vulnerable to nonlinearity that way, but otherwise nothing. Oh, and you should use the Osram ones rather than Vishay--their series resistance is much lower.
Cheers
Phil Hobbs
I've never looked in detail, but for a big PD as I move an edge across it, I just see the expected photo current. (that's DC) I think the unlighted area is just some capacitance that has to be dragged along.
George H.
On a sunny day (Tue, 20 Nov 2018 16:48:51 -0800 (PST)) it happened speff wrote in :
It is probably not what you want, but for a 3D array I'd use a cheapo analog NTSC CMOS camera and a PIC to give precise x, y position at 30 fps. I have some PIC 18F14K22 asm and ciruit if you want.
What is the application ?
Detecting a 5mm shadow at 500 m/s
~us. I hope it's not on the end of a cable. (without preamp) If it is slow, more reverse bias could help. (though I figure you know that.)
George H.
Instead of voltage mode feeding a virtual open, how about trying current mode, feeding a virtual short? That way, (theoretically) only the internal "capacitance" of the diode affects the speed; not cable capacitance, op-amp capacitance, or other bushwah. PD biasing can be almost anything you want.
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