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Interface-trap effects are analyzed in inversion-type enhancement-mode In0.53Ga0.47/ZrO2 and In0.53Ga0.47As/In0.2Ga0.8As/ZrO2 n-channel MOSFETs by comparing the measurements and the numerical device simulations of dc transfer characteristics. Device simulations can reproduce measured threshold voltages under the hypothesis that interface traps are donorlike throughout the InGaAs band gap, allowing for strong inversion operation regardless of the relatively high interface-trap density. The effects induced by the donorlike interface traps in MOSFETs having a thin cap layer interposed between gate dielectric and channel are qualitatively different from those observed in standard MOSFETs (without the cap). Increasing the donorlike trap density decreases the threshold voltage in capped devices, whereas it leaves it unchanged in uncapped ones. As a result, donorlike interface traps can explain the threshold-voltage difference observed in MOSFETs with and without the cap.