**4. Conclusions**

In this work we compared three structures obtained by sequential epitaxial growth on Si substrate. We were able to separately evaluate the contribution of AlN, AlGaN, and C-doped GaN to the vertical conduction in the GaN-on-Si stack. Also, we described the related trapping processes, showing that in presence of AlN/AlGaN layers trapping is dominated by negative charge, while in the presence of C-doped GaN, positive charge trapping also plays a role.

In addition, we investigated the breakdown voltage of the samples, indicating that a C-doped GaN layer is needed to substantially increase the breakdown strength. This is ascribed to both the insulating properties of C-doped GaN, and to the increased thickness of the vertical stack. In addition, SEM and AFM analysis were used to confirm the substantial absence of extended defects like V-pits at the surface of the C-doped layer. Finally, the temperature dependence of the critical electric field of AlN grown on a Si substrate was studied. Experimental results point out that the failure of the AlN is not related to the intrinsic breakage of the semiconductor crystal, but to a defect-related phenomenon.

**Author Contributions:** Data curation, D.B., R.P., R.K., S.B. and I.A.; writing—review and editing, A.T., M.B., M.M., C.D.S., S.B., J.D., S.D., M.G., E.M., E.Z., F.M. and G.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the project InRel-NPower (Innovative Reliable Nitride based Power Devices and Applications). This project has received funding from the European Union's Horizon 2020 research and innovation programme under gran<sup>t</sup> agreemen<sup>t</sup> No. 720527.

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