Vertical Metal–Insulator–Semiconductor (MIS) capacitors with an Al 2 O 3 thin film as a gate insulator have been fabricated on homoepitaxial GaN-on-GaN samples. The effect of the annealing treatments on the MIS characteristics has been investigated exploring two different approaches: Post-insulator-Deposition-Annealing (PDA) and Post-gate-Metallization-Annealing (PMA), ie, annealing on the bare Al 2 O 3 layer and annealing after the gate metallization deposition on Al 2 O 3. The direct comparison between PDA and PMA is crucial to understand the impact of the metal/dielectric interface quality on the behavior of the Al 2 O 3/GaN MIS capacitors. The efficacy of annealing has been monitored as a function of metal gates having different work functions: nickel (Ni), molybdenum (Mo), and tantalum (Ta). It has been found that both PDA and PMA approaches are equally able to improve the Al 2 O 3/GaN interface …