The influence of the back-gate bias on the threshold voltage and on the electron mobility of silicon trigate devices over ultra-thin-box is studied. The analysis confirms the possibility of achieving body factors higher than γ=0.1 as long as the channel width over height ratio is increased as much as possible. Also, the strong impact of the back-gate bias on the electron mobility is demonstrated using state-of-the-art scattering models for 2D confined devices.