mos body to source connection

[vikram789]

what is the need to connection source and body in mos . even if dont connect them transistor works as expected.... i am a newbie so dont mnind if it is a stupid question but many dontknow abt it i have asked many

 

[timof]

what is the need to connection source and body in mos . even if dont connect them transistor works as expected.... i am a newbie so dont mnind if it is a stupid question but many dontknow abt it i have asked many

This is not a stupid question at all. This is "basic" question, but these days many engineers and managers do not know the basics, unfortunately...

Body voltage affects device operation (for example, it affects threshold voltage Vt). Thus, for predictable device operation, body voltage should be "fixed", i.e. set to a certain value, typically - zero voltage with respect to source. There are situations (circuits, methodologies, etc.) where body voltage is set to a different level (i.e. different from Vbs=0V), but probably in majority of circuits Vbs=0V.

If it is required to have Vbs=0V, the contact to the body (i.e. contact to the well where device is located) should normally be located as close to the source contact as possible - due to a potential risk of having a large voltage drop on resistive well if there is a significant substrate/body current. The requirement is often traded off with another requirement - to have fewer well contacts to achieve higher device density.

 

[vikram789]

thanks for the info....
i read somewhere that body and source are connected so as to keep the p-n junction formed by them ( NMOS -> source= n & body =p) reversed biased as body is normally coneected to the most negative supply( in case there are more than one transistor) for NMOS, well the reason given seems plausible but i have doubt what if that p-n junction is forward biased , how will flow of charges will affect the device operation

 

[timof]

thanks for the info....
i read somewhere that body and source are connected so as to keep the p-n junction formed by them ( NMOS -> source= n & body =p) reversed biased as body is normally coneected to the most negative supply( in case there are more than one transistor) for NMOS, well the reason given seems plausible but i have doubt what if that p-n junction is forward biased , how will flow of charges will affect the device operation

Actually, if the body and source are connected together, the p-n junction formed by source/body is at zero voltage.

If source/body p-n junctions get forward biased (e.g. due to substrate current and resulting voltage drop on the p-well), this leads to the change of device Vt (lowering), and more importantly, this may lead to turn-on of the parasitic bipolar transistor (source/body/drain), device latch-up, or overheating and destruction.