Trap states at the dielectric/GaN interface of AlGaN/GaN-based metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) can cause threshold voltage (V_th) instability especially under positive gate bias stress. Herein, the influence of O₂ plasma surface preconditioning (SPC) before the atomic layer deposition of the Al₂O₃ gate dielectric and of N₂ postmetallization anneal (PMA) after gate metallization on the Al₂O₃/GaN interface quality is investigated. The interface is characterized by multifrequency capacitance–voltage measurements which show a smaller frequency dispersion after the employment of SPC and PMA treatments with a reduction of the interface trap density D_it to a value in the order of 2 × 10¹² cm⁻² eV⁻¹ near the conduction band edge. The effectiveness of SPC and PMA is demonstrated in Al₂O₃/AlGaN/GaN MIS-HEMTs by pulsed current–voltage measurements which reveal improved V_th stability.