Abstract Ga₂O₃ fin field-effect transistors (FinFETs) were fabricated on β-Ga₂O₃ (010) substrates, which had on-axis (100)-plane gate sidewalls treated by nitrogen radical irradiation. The typical FinFET with a fin width (W _fin) of 400 nm demonstrated decent on-state device characteristics such as a low specific on-resistance of 6.9 mΩcm², a high drain current on/off ratio of over 10⁹, and a subthreshold slope of 82 mV/decade. The threshold voltage (V _th) of the FinFETs increased with decreasing W _fin, and enhancement-mode operation with V _th > 0 V was achieved for W _fin < 800 nm.