More megapixels, better photos: Fact or fiction?

Started Feb 7, 2008 | Discussions thread
Eric Fossum
Eric Fossum Senior Member • Posts: 1,376
Re: So many misinformed replies

ejmartin wrote:

Why do you think read noise goes down with pixel size in the examples
I listed? It's because whatever is counting the electrons has a
smaller sample to deal with, and thus makes a smaller counting error.

This is not read noise. Read noise is signal independent and is the noise contributed by all the elements in the signal chain. If the same transistor and signal chain is used by two different pixels, one large, one small, then the read noise will be the same between the two pixels.

If read noise is decreasing, it is probably because the pixel is being reengineered to reduce noise, although changed operating voltages can also affect the noise.

Right now it is thought (I think) that the dominant noise source in the pixel is residual 1/f noise from the the source-follower transistor. Reducing the number of traps in the structure (e.g. buried channel) or ability of the traps to capture carriers (e.g. p-channel) can reduce noise. As signal levels shrink due to smaller pixels this becomes increasingly important.

-Eric

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