The carrier mobility in ultrathin SOI MOSFETs with thin SiO2 gate oxide is investigated by comparing the front and back channels. The front-gate split C V method is used at large substrate voltages to determine the carrier density and mobility in the back channel. The implementation of the split CV technique in ultrathin SOI films is described. This method is also efficient for determining the threshold voltage of the back channel. The accuracy as well as the limitations of the technique are discussed. Systematic experiments are presented for advanced n-channel and p-channel SOI MOSFETs. The results confirm that the mobility in the front channel is smaller than that in the back channel. (c) 2007 Elsevier Ltd. All rights reserved.