The impact of silicon-on-insulator (SOI) thinning on electrical characteristics is investigated for various SOI impurity concentrations, and for advanced structures with TiN gates and HfO2 gate dielectrics. It is experimentally and theoretically verified that the SOI film thickness that influences the quantum-confinement effect when the thickness decreases, clearly depends on the impurity concentration of SOL However, once the quantum-confinement effect of the channel associated with SOI thinning occurs, the subband structure depends weakly on the SOI impurity concentration. Furthermore, the mobility in ultra-thin SOI-MOSFETs with HfO2 gate dielectrics and TiN gates is investigated. It is shown that the mobility of both the front and back channels is reduced, particularly, in the low electron density region. In addition, the mobility exhibits considerable variations in this region. These variations do not correspond to the variations in SOI thickness or gate-oxide thickness. The results suggest that they are due to the variations in the Coulomb scattering centers. (C) 2007 Elsevier Ltd. All rights reserved.