International audience; The rapidly increasing power demand, downsizing of power electronics and material specific performance limitation of silicon has led to the development of AlGaN/GaN heterostructures for high power applications. In this frame, emerging AlxGa1-xN channel based heterostructures show promising features for next generation of power electronics. In this work, we propose the study of breakdown field variation through the AlGaN channel HEMTs-on-Silicon with various Al composition. The fabricated devices exhibited remarkable buffer breakdown field > 2.5 MV/cm for sub-micron heterostructures grown on silicon substrate. Furthermore, we also experimentally demonstrate that Al-rich AlGaN channel enable both boosting the 3-terminal transistor breakdown voltage and also benefiting from a superior thermal stability.