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Design >> Analog Design >> threshold voltage of MOSFET / own well
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Message started by ontheverge on Sep 28^th, 2010, 2:31pm

Title: threshold voltage of MOSFET / own well
Post by ontheverge on Sep 28^th, 2010, 2:31pm

Hi,

Placing MOSFETs in their own well is effective in eliminating body effect. my quesiton is, is this method widely used in practical? or it's just for simplifying the design?

thanks,
Stephen

Title: Re: threshold voltage of MOSFET / own well
Post by raja.cedt on Sep 28^th, 2010, 9:53pm

hi,
many times people prefer transistor in separate well but mainly to isolate from the substrate noise due high digital stuff and this is so called deep N-well process.

Thanks,

Title: Re: threshold voltage of MOSFET / own well
Post by Alexandar on Sep 29^th, 2010, 1:45am

Depends on the foundry as well. If their spacing rules are very high, you make seperate wells only when you think it is really necessary (noise from supplies or other analog req.). When the spacing rules are less tight, the area penalty is smaller, so you might apply it to less critical blocks as well.

Title: Re: threshold voltage of MOSFET / own well
Post by vivkr on Sep 29^th, 2010, 2:32am

ontheverge wrote on Sep 28^th, 2010, 2:31pm:

Hi,

Placing MOSFETs in their own well is effective in eliminating body effect. my quesiton is, is this method widely used in practical? or it's just for simplifying the design?

thanks,
Stephen

Depending on your process technology, this option may actually be very attractive in some cases, and sometimes even necessary. There are many reasons for tying bulk-source, some of which are:

1. Reduced body effect
2. As a result of 1, you may have better headroom and mismatch
3. Avoiding overvoltage condition in charge-pumps (voltage boosters).
4. Higher output impedance in cascoded mirrors or gain-boosted amps.

The most common situation where this is needed and usually supported by the foundry is for high-voltage design. Eliminating body effect may be desirable for a designer but body effect by itself is seldom a killer. Overvoltage on the other hand is a killer.

Vivek