In this paper, a comparative study of the early growth stages of aluminum oxide (Al2O3) thin dielectric layers deposited by thermal- (T-ALD) and plasma-enhanced atomic layer deposition (PE-ALD) on AlGaN/GaN heterostructures is presented. In particular, the evolution of the morphological features and the electrical properties of all the deposited Al2O3 layers has been investigated at the nanoscale by conductive-atomic force microscopy and correlated to electrical measurements on metal–insulator–semiconductor capacitors. The different insulating characteristics and their relative evolution upon increasing the film thickness are an indication of the occurrence of two different ALD growth mechanisms. As a matter of the fact, chemical characterization by X-ray photoelectron spectroscopy also corroborated the nanoscale electrical results and provided evidence that the PE-ALD process occurs under an ideal layer-by …