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Bulk of NMOS tied to different potential instead of ground?

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fazan83

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Guys what does bulk of mosfet mean?
Is it base of mosfet?

I got this simple question from my lecturer but I am in doubt to answer it.
" Can the Bulk of NMOS tied to different potential instead of ground?
If yes why? If no why?"

If NMOS bulk mean NMOS base then why we want to tied it to ground as it will always turn off.

I had search through the web but couldn't find the answer that I need.

Please help guys.
 

In general single well p type substrate process. the bulk of NMOS must connect to ground, it can not tired to different potential. Only the bulk of PMOS can tired to different potential. Because PMOS are made in N-Well, which is the bulk of PMOS. and N-Well are separated from each other.
 

Re: Bulk of NMOS tied to different potential instead of grou

RDRyan said:
In general single well p type substrate process. the bulk of NMOS must connect to ground, it can not tired to different potential. Only the bulk of PMOS can tired to different potential. Because PMOS are made in N-Well, which is the bulk of PMOS. and N-Well are separated from each other.
Click to expand...

But can you please see this link:
**broken link removed**

Base on this link we can connected the bulk to other potential instead of ground.

Why you said it NMOS bulk cannot tied to different potential than ground?
Can you please explain more on this?
I don't really understand.
 

The bulk of NMOS are common. but PMOS are not. If one of the NMOS's body connects to its source which has a potential higher than ground, all the bulk of NMOS will connect to that NMOS's source.
and other NMOS's source may connect to ground. so some NMOS may have its PN junction forward biasd, which is not allowed in the circuit.

Ryan
 

Re: Bulk of NMOS tied to different potential instead of grou

RDRyan said:
The bulk of NMOS are common. but PMOS are not. If one of the NMOS's body connects to its source which has a potential higher than ground, all the bulk of NMOS will connect to that NMOS's source.
and other NMOS's source may connect to ground. so some NMOS may have its PN junction forward biasd, which is not allowed in the circuit.

Ryan
Click to expand...

Thanks for your explanation.
 

Re: Bulk of NMOS tied to different potential instead of grou

suppose i have a single NMOS in my substrate then connecting it to a negative potential will reduce the threshold voltage... am i correct...
 

Re: Bulk of NMOS tied to different potential instead of grou

lordsathish said:
suppose i have a single NMOS in my substrate then connecting it to a negative potential will reduce the threshold voltage... am i correct...
Click to expand...

It seems that the threshold voltage will increase.
 

Re: Bulk of NMOS tied to different potential instead of grou

when a negative voltage is applied to the substrate of the nmos it repels the electrons which are minority carriers in the substrate towards the channel... so the channel can be formed easily.... so the threshold voltage has to reduce...
 

Re: Bulk of NMOS tied to different potential instead of grou

If you tied the bulk of NMOS to negative potential then your threshold will increase as you would be required to apply more positive voltage on gate to compensate for this decrease in p-substrate potential.(Your depletion region in the channel and across diffusion regions becomes wider hence the potential and inversion channel will not be form until and unless you apply same potential on gate, i.e the threshold)
 

Re: Bulk of NMOS tied to different potential instead of grou

I couldn get you... if the depletion region increase why do we need to apply more voltege to form inversion...
 

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