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We report the modification and control of threshold voltage in enhancement and depletion mode AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors through the use of in-situ fluorine doping of atomic layer deposition Al2O3. Uniform distribution of F ions throughout the oxide thickness are achievable, with a doping level of up to 5.5 × 1019 cm-3 as quantified by secondary ion mass spectrometry. This fluorine doping level reduces capacitive hysteretic effects when exploited in GaN metal-oxide-semiconductor capacitors. The fluorine doping and forming gas anneal also induces an average positive threshold voltage shift of between 0.75 and 1.36V in both enhancement mode and depletion mode GaN-based transistors compared with the undoped gate oxide via a reduction of positive fixed charge in the gate oxide from +4.67 × 1012 cm-2 to -6.60 × 1012 cm-2. The application of this process in GaN based power transistors advances the realisation of normally off, high power, high speed devices. [ABSTRACT FROM AUTHOR]