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bulk of NMOS tie to different potential instead of ground?

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rock888ford

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nmos bulk ground

can the bulk of NMOS tie to the different potential instead of ground ?if yes?why?if no why?
 

nmos potential

Yes u can connect it like that if your foundry supports Tripple well process. ie in a nwell process you have to create a nwell first and a pwell in this nwell to act as Substrate to this type of Nmos. You can connect now this Pwell which acts as the substrate to your Nmos any where. Because of lot of Wells your layout area increses.
 

nmos tie bulk and source

you can connect bulk to different potentials as per your requirement of Vth.
As Vth depends on VBs
for + ve bulk voltage, Vth will increase. Whereas for -ve bulk voltage Vth will decrease.
 

connect bulk to ground

If u connect the bulk to same potential as the source(in nmos),then your threshold voltage wil be Vt0.. If u connect it to different potential,it wil vary according the formula,

Vt=Vt0+gamma*(sqrt(Vsb+2*phi)-sqrt(2*phi)),where phi is the surface potential parameter and gamma is the body effect parameter..

So depending on where u connect ur bulk,ur threshold varies..
 

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