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I want to understand what a comment means, and I couldn't ask in a comment. In this question, someone asked why use an op-amp with a photodiode rather than just a resistor. The answers and comments were great, and in a particular a comment by @john-doty said, "For further improvement, put negative bias on the diode's anode. This reduces the capacitance while providing a stronger field to sweep carriers out of the depletion, for better speed and linearity. – John Doty Jul 17 at 14:52"

Would someone please expand on that? Specifically, what changes to the circuit schematic does this imply, and what does "... providing a stronger field to sweep carriers out of the depletion ..." mean? I have taken college physics, so please dig right in. I want to understand what's going on so that I can fine-tune with less guessing.

Thank you!

The circuit in discussion: photodiode sensor with op-amp TIA

MachineMaker
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what changes to the circuit schematic does this imply,4

You'd connect the anode of the photodiode to a negative voltage instead of to ground.

what does "... providing a stronger field to sweep carriers out of the depletion ..." mean?

It means that with a stronger reverse bias, the field in the depletion region of the diode will be stronger, and therefore optically excited carriers (electrons and holes) will be swept out of the junction more quickly, increasing the bandwidth of the detector.

The Photon
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  • It does a lot more. For just one example, it changes the wavelengths that can generate swept charges. So, depending on the exact design details the quantum efficiency may shift to include more towards the longer end of the wavelengths than it used to accept, for example. This may, or may not, be a desired effect. – jonk Oct 07 '20 at 03:14
  • @jonk, in my experience that effect is not usually significant. Maybe if you were working right on the long-wavelength end of the responsivity curve, it would make a difference. But if you're working in the middle of the responsivity curve, reverse bias has very little effect on responsivity. – The Photon Oct 07 '20 at 17:02
  • I actually use this in devices I've worked on. I've also used other techniques to alter the response wavelengths: thin, charged gold layer on the top surface, laser dyes, etc. It matters a great deal, in fact. (This is in astronomical work I've done over the years.) – jonk Oct 07 '20 at 17:29