Bulk connection in NMOS transistor biasing

[circuitking]

Hi
I know from my circuit theory lecture.NMOS bulk is connected to ground/source terminal/most -ve voltage in ICs and in PMOS the bulk is connected to VDD/source terminal/most +ve voltage in ICs. However in FDSOI technology bulk/body can be connected to some DC voltage. I would like to know in NMOS when to connect it to ground and to some DC voltage ,advantages and disadvantages of both ways

 

[ICdesignerbeginner]

Bulk driven MOS is used for low voltage operations. It removes the threshold voltage from the signal path because signal is applied at bulk. Secondly it helps in linearity improvement as it inherently attenuates the signal.
But it increases the input referred noise. Chip area, parasitic capacitance increases.

 

[erikl]

Bulk driven MOS ... removes the threshold voltage from the signal path because signal is applied at bulk.

So you need a twin/triple well (for NMOS) or SOI process - if you want more than a single stage.

Secondly it helps in linearity improvement as it inherently attenuates the signal.

I.e. it's not suitable for amplification? And could you explain how attenuation improves linearity?

 

[timof]

Hi
I know from my circuit theory lecture.NMOS bulk is connected to ground/source terminal/most -ve voltage in ICs and in PMOS the bulk is connected to VDD/source terminal/most +ve voltage in ICs. However in FDSOI technology bulk/body can be connected to some DC voltage. I would like to know in NMOS when to connect it to ground and to some DC voltage ,advantages and disadvantages of both ways

View attachment 145379

For nMOSFET in FDSOI:

- negative body bias --> higher Vt --> lower leakage (good) and lower performance (bad)

- positive body bias --> lower Vt --> higher leakage (bad) and higher performance (good)

GlobalFoundries provides some useful material on body biasing in FDSOI:

https://www.globalfoundries.com/sit...ology-new-dimension-to-explore-the-design.pdf

https://www.globalfoundries.com/res...uccessfully-implement-22fdx-fd-soi-technology