The standard bulk MOSFET definition for effective electric field is mo
dified for SOI devices to account for nonzero electric field at the ba
ck oxide interface. The effective channel mobility in fully-depleted n
-channel SOI MOSFET's is shown to be independent of applied back-gate
bias when the modified E(eff) definition is used. The effective channe
l mobility as a function of E(eff) is also shown to be independent of
film thickness for fully-depleted devices.