Enhanced thermal performance of AlN/GaN/AlN XHEMTs on bulk AlN by suppression of phonon-boundary scattering

AlN/GaN/AlN high electron mobility transistors (HEMTs) have demonstrated exceptional potential for surpassing the electrical limitations of conventional AlGaN/GaN HEMTs. This study investigates the thermal performance of two types of AlN/GaN/AlN HEMTs with homoepitaxial AlN buffer layers grown on AlN substrates: an AlN/GaN/AlN single-crystal HEMT (AlN XHEMT) featuring a pseudomorphic/thin GaN channel and a conventional structure with a relaxed/thick GaN channel. Frequency- and time-domain thermoreflectance measurements reveal bulk-like thermal conductivity in the homoepitaxial AlN buffer layer, with negligible thermal boundary resistance at the AlN buffer/substrate interface. Consequently, Raman thermometry demonstrates that the AlN XHEMT with a thin (∼20 nm) pseudomorphically strained GaN channel exhibits better thermal performance than identical HEMT layer structures grown on a 4H-SiC substrate, despite 4H-SiC possessing a higher thermal conductivity. In addition, the AlN XHEMT exhibits a 22% lower channel temperature under 14 W/mm power density than the AlN/GaN/AlN-on-AlN HEMT that employs a thick (275 nm) relaxed GaN channel. These findings highlight that AlN XHEMTs offer not only electrical but also thermal advantages for high-power and high-frequency applications.