We introduce in this work a new quality factor called effective resistivity (rho(eff)), which is used to characterize and fairly compare the substrate resistivity of fully processed SOI wafers. The impacts on rho(eff) (and thus on microwave losses) of the bias (V-a), fixed oxide charges (Q(ox)) traps at the SiO2/Si interface (D-it), oxide thickness (t(ox)) and line geometry are quantified and discussed for the first time. Different design and technological conclusions are drawn. (C) 2004 Elsevier Ltd. All rights reserved.