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What is the advantage of using diffusion butting ?

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mberesin

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butted diffusion

Hello

I want to ask what is the advantage of using diffusion butting.
How will it work when source and bulk of transistor are on the same potential and I will place source diffusion and well tap without any space between them.
Are there any sugestions when to use butting?
 

butting tap

Whenever the Design Rules allow for it. It is wise to have the bulk tap as close as possible to the source (if they are at the same potential, of course). This minimizes the bulk control of the MOSFET.
 

Re: Diffusion butting

Thanks for the reply erikl.

Maybe i can ask this way:
Are there any situations when i shouldn't use butting (other than different potentials on source and bulk)?
 

Re: Diffusion butting

mberesin said:
Thanks for the reply erikl.

Maybe i can ask this way:
Are there any situations when i shouldn't use butting (other than different potentials on source and bulk)?
Click to expand...
No, not at all. It is the best you can do from a design aspect, from physical reasons, and from minimizing silicon area.
However, I've seen process technologies, where design rules forbid this butting. Don't know why, no design reason, could only arise from mask resolution inaccuracy, IMHO.
 

Re: Diffusion butting

Butting is not recommended in some cases (analog designs), related to the noise injected into the substrate.
If the bulk diffusion is close to the source of a transistor carrying a large current, acting as a switch, for example, the noise injected in the substrate is the same amplitude as that source. But if the substrate connection is not firmly connected to that bouncing source, but to a "stiff" supply, then the noise injected is far less.
 

Re: Diffusion butting

mario1980 said:
Butting is not recommended in some cases (analog designs), related to the noise injected into the substrate.
If the bulk diffusion is close to the source of a transistor carrying a large current, acting as a switch, for example, the noise injected in the substrate is the same amplitude as that source. But if the substrate connection is not firmly connected to that bouncing source, but to a "stiff" supply, then the noise injected is far less.
Click to expand...
Good objection! Working on micro-power analog projects, I didn' think of this, sorry! :-(
 

Re: Diffusion butting

Thank You both for the discussion.

However i don't understand problem with noise.
If I use butting i connect source with bulk with meta1.
If I don't use butting i also connect them with metal1. So both ways there will be good connection from source to substrate.
 

Re: Diffusion butting

mberesin said:
If I don't use butting i also connect them with metal1. So both ways there will be good connection from source to substrate.
Click to expand...
In such a noise-jeopardized situation, you should not connect the source and the substrate tap directly. Connect both connections by separate metal wires to a "stiff" power supply point, as Mario explained. So the noise voltage generated over the source wire connection will not be supplied to the separate (not butted) substrate tap near the source.
That's how I understood Mario, and I think it's correct.
 

Re: Diffusion butting

Ok, now I understand.

thanks.
 

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