In this letter, we investigate the scaling potential of flatband IIIV MOSFETs by comparing the mobility of surface- and buried-channel In 0.53Ga0.47As devices employing an atomic-layer-deposited Al2O3 gate dielectric and a delta-doped InGaAs/InAlAs/InP heterostructure. Peak electron mobilities of 4300 cm2/V ċ s and 6600 cm2/V ċ s at a carrier density of 3 × 1012 cm-2 were determined for the surface- and 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 × 1012 cm-2 before slowly dropping to around 2000 cm2/V ċ s at 1 × 10 13cm-2. We believe these to be world leading metrics for this material system and an important development in informing the IIIV MOSFET device architecture selection process for the future low-power highly scaled CMOS. © 2011 IEEE.