Interplay of carrier density and mobility in Al-Rich (Al,Ga)N-Channel HEMTs: Impact on high-power device performance potential

Despite considerable advancements, high electron mobility transistors (HEMTs) based on gallium nitride (GaN) channels remain largely limited to power applications below 650 V. For higher power demands, the ultra-wide bandgap semiconductor alloy aluminium gallium nitride, (Al,Ga)N, has emerged as a key contender for next-generation HEMTs. In this theoretical study, we show that Al-rich Al${}_x$Ga${}_{1-x}$N-channel HEMTs (with $x\ge 0.5$) outperform the GaN-channel counterparts at and above room temperature, across all Al compositions, $x$. This contrasts with recent theory reports which suggest that only Al${}_x$Ga${}_{1-x}$N HEMTs with high Al content ($x\ge 0.85$) offer comparable performance to GaN-channel devices. Unlike previous assumptions of a constant two-dimensional electron gas (2DEG) density across the entire composition range $x$, we show that the 2DEG density is highly sensitive to both the Al content and thickness of the individual layers in a HEMT structure. We demonstrate that the superior performance of Al-rich (Al,Ga)N-channel HEMTs is driven by a competing effect between 2DEG density and electron mobility. This work challenges the assumptions of prior studies, which can result in a significant under or overestimation of the potential of high Al content HEMTs. The insights gained from our work provide a comprehensive understanding of the trade-offs between device and material parameters, thus help to guide the design of future Al-rich ($x=0.5-1.0$) Al${}_x$Ga${}_{1-x}$N-channel HEMTs for high-power applications.