Are bigger pixels less noisy?

Started 4 months ago | Discussions thread
bobn2 Forum Pro • Posts: 69,708
Re: Are bigger pixels less noisy?

Eric Fossum wrote:

bobn2 wrote:

I had a huge great argument here with Eric on this topic some years ago. Anyhow, this is what I was saying, reworded a bit, in deference to Eric.

Thanks for deference, although to be honest I have no recollection. Hope it was a tie.

When you're looking for dependencies, you end up making some assumptions, which are likely not realistic, but if the reality carries some correlation with the assumptions, maybe you've captured 'weak dependency'.

So, let's assume that all pixels are designed the same (of course, they aren't) and we produce small pixels just by scaling large pixels down proportionately. This means that every feature of the pixel gets smaller (and also, another discussion, that it's fill factor stays the same). The input referred read noise depends on the electronic noise from the pixel source follower and downstream and the conversion gain of the pixel. In turn the conversion gain is inversely proportional to the capacitance of the SF gate and floating diffusion. If these are scaled down the capacitance goes down, the CG goes up and the read noise is reduced.

The limiting noise in SFs these days is often 1/f noise. 1/f noise goes like 1/(gate area)^x where x is about 1 and x depends on a lot of factors it seems. So you get improved CG and worse output-referred voltage noise as the SF gate area shrinks. CG depends on other parasitic capacitances too so CG improvement is not proportional to SF-gate-area shrink so it could go the other way -> smaller SF gates gives more noise.

I understand you are trying to simplify things here but in the simplification you might get the right answer for the wrong reasons.

Yes, I understand it could go the other way, but we are talking 'weak dependencies' here, so the dependency is in the end about the aggregated chances of it going one way rather than another. Plus, in camera applications maybe a fair part of the read noise is downstream noise, not just the SF, so it's not always the case that the limiting noise in the SF is the one that determines the overall result - and the CG acts on all noises.

The general problem with trying to explain observed trends is that any analysis is bound to be simplistic, because the trend is a whole load of data points which are distributes all around the trend line, and any individual point might be go against the trend.

I think JACS observation aligns with mine, and the conversion gain reason seems to me the most likely, but if there are better suggestions, of course I'll defer to greater knowledge than mine.

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