β-phase gallium oxide (Ga₂O₃) is an emerging ultrawide bandgap (UWBG) semiconductor (E_G ∼ 4.8 eV), which promises generational improvements in the performance and manufacturing cost over today's commercial wide bandgap power electronics based on GaN and SiC. However, overheating has been identified as a major bottleneck to the performance and commercialization of Ga₂O₃ device technologies. In this work, a novel Ga₂O₃/4H-SiC composite wafer with high heat transfer performance and an epi-ready surface finish has been developed using a fusion-bonding method. By taking advantage of low-temperature metalorganic vapor phase epitaxy, a Ga₂O₃ epitaxial layer was successfully grown on the composite wafer while maintaining the structural integrity of the composite wafer without causing interface damage. An atomically smooth homoepitaxial film with a room-temperature Hall mobility of ∼94 cm²/Vs and a volume charge of ∼3 × 10¹⁷ cm^-3 was achieved at a growth temperature of 600 °C. Phonon transport across the Ga₂O₃/4H-SiC interface has been studied using frequency-domain thermoreflectance and a differential steady-state thermoreflectance approach. Scanning transmission electron microscopy analysis suggests that phonon transport across the Ga₂O₃/4H-SiC interface is dominated by the thickness of the SiN_x bonding layer and an unintentionally formed SiO_x interlayer. Extrinsic effects that impact the thermal conductivity of the 6.5 μm thick Ga₂O₃ layer were studied via time-domain thermoreflectance. Thermal simulation was performed to estimate the improvement of the thermal performance of a hypothetical single-finger Ga₂O₃ metal-semiconductor field-effect transistor fabricated on the composite substrate. This novel power transistor topology resulted in a ∼4.3× reduction in the junction-to-package device thermal resistance. Furthermore, an even more pronounced cooling effect is demonstrated when the composite wafer is implemented into the device design of practical multifinger devices. These innovations in device-level thermal management give promise to the full exploitation of the promising benefits of the UWBG material, which will lead to significant improvements in the power density and efficiency of power electronics over current state-of-the-art commercial devices.

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