Bulk fixing CMOS

Thread Starter

Luiz Fernando Vieira

Joined Aug 24, 2015
Hello everyone I'm having trouble finding the material that talks about CMOS regarding bulk fixation. I know that the bulk is welded in well n (n-well) or well p (p-well) depending on its structure, because it is welded, because it is more doped so the flow of electrons is easier this way a behavior metallic. But I can't find material that specifically talks about this in a book or booklet or a reliable website in the academic area. I would like a material that talks about it.

I do not know if it is a translation error that I am making of the following words, gluing, welding, fixing, nailing.



Joined Mar 30, 2015
Welded isn't a correct descriptor. The P or N region is called diffusion because it's diffused into material of the opposite polarity.

What do you mean by fixation? I've never heard the term used to describe diffusion.


Joined Mar 31, 2012
I think what you are asking about is basically what voltage should the bulk (i.e., transistor body) be tied to.

The bulk is generally a lightly doped region in which the transistors are made. It forms a p-n junction with the transistor structures and you want that junction to be reverse biased so that current doesn't flow in it. So you want the body of NFETs to be at a low voltage and the body of PFETs to be at a high voltage. With single well processes, say an N-well process, all of the NFETs share the same body, namely the bulk of the entire die, and so all of their bodies will be at the same potential. For NFETs in which the source is not connected to the bulk, this means that the body and the source are not at the same voltage and this changed the transistor characteristics (this is known as the body effect). It also means that there is the potential for crosstalk between transistors via bulk currents. For PFETs you have the option of putting each PFET in its own N-well and tying the well to the transistor source, thus greatly reducing both of these issues -- at the expense of a considerable increase in circuit area.

You can even exploit the availability of a fourth terminal on a PFET and use it to electrically control the transistor characteristics. This is known as body-bias modulation.