When we use 3 terminal and when we use 4 terminal Mosfets in a Design.?

[vlsi123]

Hi,

As per my understanding we can have 3 terminal or 4 terminal mosfets in our Design. Some designs uses only 3 terminal mosfets where we don't want the connection of Bulk ( I can say Substrate connection) . In most of the scenarios we need bulk connection means 4 terminals. So please can anyone tell me when do we need to use 3 terminal Mosfet and when do we need to use 4 terminal Mosfet in designs.?

I know p type substrate for nmos ( connected to ground) and n type substrate for pmos ( connected to vdd) . But i am not clearly aware how is this useful in a design.? Why it is of not much importance in some design where we don't need substrate connection.?

I have seen in one thread here that lateral parasitic diodes are formed. To avoid that we use substrate connection.? Is there any other reason in addition to it.?

If that is the case why don't we need this substrate connection in some designs.?

Thanks
vlsi123
Thanks,
Chaitra

 

[srizbf]

this what i understand:

a 3 terminal mosfet have default connection of source to substrate.

a 4 terminal one have separate connection points.

 

[keith1200rs]

and a 3 terminal PMOS has the bulk connected to a global node, usually named VDD. They are all really 4 terminal, it is just a convenience to avoid drawing the fourth connection all the time when it is often not used.

Keith.

 

[checkmate]

Read up on body effect. Some designs make use it for sub-threshold operation. Some want to eliminate it by shorting the body to source, especially in cases where you do not have enough headroom.

 

[vlsi123]

Hi,

Thanks....
I found one univeristy handout which explained this concept neatly.
Here it is
https://www.ittc.ku.edu/~jstiles/312/handouts/The Body Effect.pdf