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Using a full-band and atomistic approach based on the nearest-neighbor tight-binding model and the nonequilibrium Green function formalism, (111)/(110) GaSb, (100)/(110) strained-Si, and (100)/(100) In0.53Ga0.47As n-type double-gate ultrathin-body field-effect transistors designed according to the ITRS specifications for 2020 are simulated in the ballistic limit of transport and with electron-phonon scattering. It is found that, at an equivalent oxide thickness of 0.59 nm, the GaSb device offers the highest ballistic ON-current at a fixed OFF-current, due to the projection to the Γ point of bands originating from the bulk L-valley and possessing a low transport effective mass. It is followed by the strained-Si FET and, finally, the In0.53Ga0.47As FET, the latter suffering from its small density of states in the channel despite very high electron velocities. However, when electron-phonon scattering is taken into account, the presence of multiple energy subbands, as in GaSb and strained Si, increases the probability of backscattering for electrons; thus, the current of these devices does not exceed that of the In0.53Ga0.47As FET by more than 13 %.