The rapidly increasing power demand, downsizing of power electronics and material specific performance limitation of silicon have led to the development of AlGaN/GaN heterostructures for high power applications. In this frame, emerging Al x Ga 1-x N channel based heterostructures show promising features for the 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 electric field > 2.5 MV/cm for sub-micron heterostructures grown on silicon substrate. Furthermore, we also experimentally demonstrate that Al-rich AlGaN channel enables both boosting the 3-terminal transistor breakdown voltage and benefiting from superior thermal stability.