Effects of the gate recess structures on the DC performances were investigated in 0.1-μm metamorphic high-electron-mobility transistors. Narrow gate recess structure showed significantly enhanced DC characteristics compared to wide gate recess structure in terms of drain-source saturation current increasing from 440 to 710 mA/mm and extrinsic transconductance increasing from 420 to 910 mS/mm. We propose that the observed variations in DC characteristics are due to the deep-level acceptor-type interface defects formed between the silicon-nitride passivation layer and the Schottky barrier layer. We performed hydrodynamic model simulation to verify the proposed mechanism, and the calculated result showed a good agreement with the experimental observation.