We present a comprehensive theoretical investigation of the quantum confinement limited mobility in the Si1− x Ge x -channel gate-all-around nanosheet field effect transistor for 5-nm node. The study encompasses physics-based quantum mechanical models both for P and NMOS with specified channel/wafer orientations and channel thicknesses: 1) k.p model with Poisson solver for band structures, bandgap variations, and confined charge distributions; 2) Kubo-greenwood model for low field mobility with considering surface roughness and stress; 3) multisub-band Boltzmann transport equation based on a state-of-the-art phase space approach is employed to evaluate device IV characteristics; and 4) the threshold voltage ( V T ) variations with different channel/wafer orientations are also evaluated. Our simulation study shows that {110} wafer Ge channel can be an attractive option for 5-nm node pMOS, and Si is still promising for nMOS due to strong quantum confinement in Ge channel.