In this work, the conduction mechanisms at the interface of AlN/SiN dielectric stacks with AlGaN/GaN heterostructures have been studied combining different macroscopic and nanoscale characterizations on bare materials and devices. The AlN/SiN stacks grown on the recessed region of AlGaN/GaN heterostructures have been used as gate dielectric of hybrid metal–insulator–semiconductor high electron mobility transistors (MISHEMTs), showing a normally-off behavior (Vth = +1.2 V), high channel mobility (204 cm2 V–1 s–1), and very good switching behavior (ION/IOFF current ratio of (5–6) × 108 and subthreshold swing of 90 mV/dec). However, the transistors were found to suffer from a positive shift of the threshold voltage during subsequent bias sweeps, which indicates electron trapping in the dielectric stack. To get a complete understanding of the conduction mechanisms and of the charge trapping …