**4. Conclusions**

This work presents the fabrication and characterization of thin and thick channel AlGaN/GaN HEMTs grown on AlN/sapphire templates. Lateral buffer breakdown voltage assessment of the thin channel reveals a remarkable breakdown field of 5 MV/cm for short contact distances, which is far beyond the theoretical limits of a GaN-based material system. Furthermore, fabricated transistors are fully functional with low leakage current and low on-resistance. The use of a sub-10 nm ultrathin GaN channel may not be a limiting factor for the breakdown mechanism of transistors. Temperature measurements up to 300 ◦C show that the AlN buffer enables increased temperature stability of GaN-based transistors. When combined with a thin channel, the related transistors may deliver lower channel temperatures as compared to devices with thicker channels. Proper passivation and associated processing will allow us to take advantage of the properties offered by AlN-based devices, providing both low resistances and high voltages well above 1 kV.

**Author Contributions:** Device design, fabrication and characterization, I.A., R.K. and F.M., TEM images, C.B., Temperature measurements, J.P., C.M., R.C., Growth, Y.C., Writing-Original Draft Preparation, all co-authors.

**Funding:** This work is supported by the French RENATECH network and the French National gran<sup>t</sup> ANR-17-CE05-00131 within the project called BREAKUP and ANR-11-LABX-0014 representing GaNeX: the "Investissements d'Avenir".

**Conflicts of Interest:** The authors declare no conflict of interest.
