Body substrate of transistor?

[zahrein]

Guyz,

I ve been asked why , the body of PMIS is connected to VDD and the body of NMOS is connected to ground?..

I know the answer is like becoz of boy effect. I dont know on how to answer in elctronic ways...Its quite difficult for me to understand. Can anybody explain...

 

[DReynolds]

Re: Body or substrate of transistor?

Zahrein, I am not sure from your post how detailed an explaination you want...

One way to think of it as "back gate", ie like another input and if you increase the source to body voltage (for nmos) this back gate works against the voltage you put on the gate terminal.


hope this helps

David Reynolds

 

[zahrein]

a lttle bit...BUt anyone can explain more.. tq..

 

[ambreesh]

Man look at the 3D structure of the device.
We have back to back connected diodes, you donot want any of them to be forward biased.

Example PMOS

Top to Bottom
Pdiffusion
Nwell
Psubstrate.

Now we have diode between P+and Nwell to reverse bias it Nwell has to be at same or higher potential than the P+ diffusion
Similar for diode between Nwell and P substrate

Hope i am clear

 

[nittinsharma80]

Hi
electronic ways??
looks like you are very novice in the MOS fiels. but you your self have to develop your fundamentals.
just try to grasp the MOS operation and the effect of Vsb (source - bulk voltage) on Vth.
Also try to think of the effect of change in Vth on the MOS output current Id.
search for body bias. why and how need to be found out and once you do it, the answer of your qn will be clearly visible to you.
best of luck.

 

[ceyjey]

Hi,
First, in general MOS process, the source and drain are isolated with PN junctions. In order to ensure proper operation of MOS device, the PN junctions have to be reverse biased.
Secondly, as you know, NMOS has a P-type substrate with N-type source and drain, while PMOS has a N-type substrate with P-type source and drain. In order to reverse bias the PN junction, you have to connect the substrate of NMOS to GND and the substrate of PMOS to VDD.
You might probably argue that one does not have to do that to achieve a reverse bias. Yes, as long as you maintain the voltage of N side higher than the P side, you can achieve a reverse bias. But the problem is in a representative N well process, the P type substrate is the universal substrate of all NMOS devices on a single die. You simply don't know at what voltage will the PN junction safely biased, except the lowest available voltage GND. The same applies in N type substrate.
Hope this helps

regards
ceyjey