Abstract <p>In this study, we employ Technology Computer Aided Design (TCAD) simulations to understand the impact of Al composition (<i>x</i>) in Al_<i>x</i>Ga_{(1–<i>x</i>)}N buffer and barrier layers on GaN High Electron Mobility Transistors (HEMTs) on silicon substrates. We uncover the critical role of Al composition in enhancing the quantum well at the Al_<i>x</i>Ga_{(1–<i>x</i>)}N/GaN interface, effectively confining electrons and directing the two-dimensional electron gas (2DEG). Our simulations yield impressive results, including a high electron mobility of 1460 cm²/Vs and an electron density of 7.5 × 10¹⁹ cm^–3. Moreover, as we increase the Al composition in the barrier layer from 0.15 to 0.30, we observe a substantial rise in the breakdown voltage, a crucial parameter for optoelectronic and quantum devices. Specifically, the breakdown voltage reaches 618 V, accompanied by a threshold voltage of 3 V and a drain current of 0.51 A/mm at 0 V gate voltage. These findings not only advance our understanding of Al_<i>x</i>Ga_{(1–<i>x</i>)}N/GaN heterostructures but also underscore their potential in driving the improvement of future optoelectronic and quantum devices, making them highly relevant to the field of optoelectronic and quantum devices.</p>