Main scattering mechanisms affecting electron transport in MOS/SOI devices are considered within the quantum-mechanical approach. Electron mobility components (i.e., phonon, Coulomb and interface roughness limited mobilities) are calculated for ultrathin symmetrical DG SOI transistor, employing the relaxation time approximation, and the effective electron mobility is obtained showing possible mobility increase relative to the conventional MOSFET in the range of the active semiconductor layer thickness of about 3 nm