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We report on the impact of H2 high-pressure annealing (HPA) onto In0.7Ga0.3As MOSCAPs and quantum-well (QW) MOSFETs with Al2O3/HfO2 gate-stack. After HPA with process condition of 300 °C, H2 ambient and pressure of 20 atm, we observed notable improvements of the capacitance–voltage (CV) characteristics in InGaAs MOSCAPs with Al2O3/HfO2 gate-stack, such as reduction of equivalent-oxide-thickness and less frequency dispersion in the accumulation region. There was 20% improvement of the interfacial trap density ( $D_{{\rm {it}}})$ . Then, we incorporated the HPA process into the fabrication of sub-100-nm In0.7Ga0.3As QW MOSFETs, to investigate the impact of HPA process. After HPA process, the device with $L_{g} = 50$ nm exhibits improved subthreshold-swing (SS) = 105 mV/decade, in comparison with SS = 130 mV/decade for the reference device without HPA process. Finally, we carried out reliability assessment under a constant-voltage-stress (CVS), and it turns out that the HPA process was effective in mitigating a shift of threshold voltage ( $\Delta \text{V}_{T})$ during the CVS. These are attributed to the effective passivation of oxide traps in the high- $k$ dielectric layer and interfacial traps, after HPA process in the H2 ambient.