aaron_do
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Hi,
Is the body source or body drain voltage going above 0 V? If not, then its unlikely the diode is turning on. Most likely you are just looking at the current through the junction capacitances. You can check by lowering your input frequency to a very small number. If the body current is still there, then its not the caps.
If it is the caps, then you need to realize that the switch has an inherent trade-off between isolation and insertion loss. If you want better isolation, you need to reduce your switch width. This will of course degrade your insertion loss. You just need to decide if the isolation is good enough for you. One more thing which I mentioned before, if you really want good isolation, you need a shunt branch too (see below).
INPUT------OFF Transistor--------OUTPUT | | | ON Transistor | | | GND
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