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I'm in a CMOS class and we are currently going through the second order effects of a NMOS. I have a few questions.

When the body isn't connected to the source, the threshold voltage increases. Meaning as VGate increases, Vsource will eventually level off.

And when the body is connected to the source the threshold voltage stays constant, meaning as VGate increases, VSource increases as well.

What I do not understand is why isn't the body always connected to the source? Is there a downfall from always having the two connected?

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A P-type substrate is typically used in a CMOS process. Without some method of isolation (extra fabrication steps -> higher cost), the nFETs on a die will all share the same body. Therefore connecting the source of all of the nFETs to the body could cause an undesired connection between the sources of several nFETs. Take this circuit below as an example:

schematic

simulate this circuit – Schematic created using CircuitLab

In this circuit, both FETs have their sources connected to their bodies. Using a P-type substrate as a shared body would result in M1's drain and source being shorted.

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    \$\begingroup\$ Using a P-type substrate as a shared body would result in M1's drain and source being shorted so therefore you can only do this in a triple-well process. In a non-triple well process the body of M2 must be connected to ground. \$\endgroup\$ Commented Sep 18, 2016 at 10:45

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