β-phase gallium oxide (β-Ga₂O₃), emerging as an ultra-wide bandgap semiconductor, suffers from negative threshold voltage (V_th) characteristics, which only allow depletion-mode (D-mode) operation; however, enhancement-mode (E-mode) operation is preferred to ensure fail-safe operation and simplify circuit topologies. Therefore, in this study, the V_th is controlled via remote fluorine plasma treatment in β-Ga₂O₃ metal–insulator–semiconductor field-effect transistors (MISFETs). Under the top-gate modulation, the V_th of the fluorinated β-Ga₂O₃ MISFET was positively shifted by +4 V, exhibiting a high on/off ratio (∼10⁷) and low sub-threshold swing (175 mV dec⁻¹). Under the double-gate modulation, the E-mode β-Ga₂O₃ MISFET was demonstrated, where the V_th was estimated to be +2.2 V. The obtained results suggest that the fluorine plasma treatment is an effective method to control the V_th of the β-Ga₂O₃ FETs from D-mode to E-mode, pointing out monolithic integration of β-Ga₂O₃ transistors for future smart power electronics.