body effect
The threshold voltage of a MOSFET is affected by the voltage which is applied to the back contact. The voltage difference between the source and the bulk, VBS changes the width of the depletion layer and therefore also the voltage across the oxide due to the change of the charge in the depletion region. This results in a difference in threshold voltage which equals the difference in charge in the depletion region divided by the oxide capacitance, yielding:
dVth = ?2*?s*Na*q / Cox * ( ?2*?f-Vsb - ?2*?f )
The only way to avoid this is to use an "active well" process where you can connect the well to the source, forcing thus Vsb=0.
Hope this helps,
LucaS.