This paper presents a detailed experimental study of the electrical characteristics of long-channel ultrathin body SOI MOSFETs with standard and thin buried oxides and high-k gate dielectric, using an analysis of the transconductance, gate-to-channel capacitance and mobility behaviors at different back-gate biases. The emphasis is on the evolution of the effective mobility when shifting the conduction channel in the film from front to back interface, and oil the comparison between the two BOX thicknesses It is found that the back-channel mobility significantly exceeds the front-channel mobility, which is presumably related to strongly different Coulomb scattering at the two interfaces, being in agreement with previously published experimental Studies. Furthermore, the back-channel mobility is found to be the same for thick and thin BOX This strongly suggests that BOX thinning does not degrade the quality of the back interface. The observed effect of much higher back-channel mobility. which is retained for the thin BOX, could find application for the additional improvement of the device performance, when adjusting the threshold voltage via back-gate bias. Adequate mobility interpretation is then required as a varying combination of front and back-channel mobilities (C) 2009 Elsevier Ltd All rights reserved