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Why not PMOS switch tied to the nwell contact?

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Why is one of the PMOS switch terminals not used with nwell bulk contact?
Most of pmos switches I have seen had their bulks' were tied to the substrate contacts.

Why not??
 

You cannot tie a switch's bulk to drain or source. Simply, because for latchup reasons the PMOS device bulk has to be at the highest supply available in the circuit ( otherwise the bulk-source or bulk-drain junction may be forward biased ).

So if you tie the bulk to source or drain, you have no guarantee that the bulk will be at highest potential.
 

Every pMOSFET is in its n-well. It means every pMOSFET has its n-well, and this is its bulk (substrate, body). Connecting the source to the bulk is a kind of simplification. When VSB=0 ==> VT=VT0 and there is no body effect. It is possible if you properly bias the circuit. In reality, MOSFET is a 4-terminal device, and only sometimes the source may be tied to the bulk. In pMOSFET you may also use the bulk as a second gate to control the inversion. It is possible to use body-driven pMOSFETS as the input devices of the differential pair, and so on.
a.
 

PMOS bulk can be connected to the source to reduce the Vt
 

If you connect bulk to source you will get diode between source and drain.
In this case source-drain voltages cannot differ more than 0.7V for some signal combination.
It is not acceptable for switch.
 

When you consider this question, just to make sure the bulk is tied to the highest potential and it is already enough for your reasoning.
If you have considered it's highest potential, you can think of if this connection will affect VT, affect leakage or affect noise, whatsoever.
 

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