In this paper, the evolution of the electrical behaviour of GaN and AlGaN materials after high-temperature annealing and thermal oxidation is discussed. In particular, annealing above 1100 C, required for electrical activation of implanted species, increases the surface state density, reducing the metal/GaN Schottky barriers and increasing the leakage current. On the other hand, the thermal oxidation at 900 C of AlGaN/GaN heterostructures showed the formation of a thin oxide layer, which can be able to passivate surface defects and/or can serve as inter-device isolation. However, a decrease of the sheet carrier density in the two dimensional electron gas (2DEG) was observed when the material is subjected to such high thermal budgets. The results are discussed considering the possible optimizations and applications to GaN-devices technology.