In this letter, we investigate the scaling potential of flatband III-V MOSFETs by comparing the mobility of surface and buried-channel In 0.53 Ga 0.47 As devices employing an atomic layer-deposited Al 2 O 3 gate dielectric and a delta-doped InGaAs/InAlAs/InP heterostructure. Peak electron mobilities of 4300 cm 2 /V · s and 6600 cm 2 /V · s at a carrier density of 3 × 10 12 cm -2 were determined for the surfaceand buried-channel structures, respectively. In contrast to similarly scaled inversion-channel devices, we find that the mobility in surface channel flatband structures does not drop rapidly with the electron density, but rather high mobility is maintained up to carrier concentrations around 4 × 10 12 cm -2 before slowly dropping to around 2000 cm 2 /V · s at 1 × 10 13 cm -2 . We believe these to be world leading metrics for this material system and an important development in informing the III-V MOSFET device architecture selection process for the future low-power highly scaled CMOS.