  Ulta-wide bandgap semiconductors based on $\beta$-Ga$_2$O$_3$ offer the
potential to achieve higher power switching performance, efficiency, and lower
manufacturing cost than today's wide bandgap power semiconductors. However, the
most critical challenge to the commercialization of Ga$_2$O$_3$ electronics is
overheating, which impacts the device's performance and reliability. We
fabricated a Ga$_2$O$_3$/4H-SiC composite wafer using a fusion-bonding method.
A low temperature ($\le$ 600 $^{\circ}$C) epitaxy and device processing
approach based on low-temperature (LT) metalorganic vapor phase epitaxy is
developed to grow a Ga$_2$O$_3$ epitaxial channel layer on the composite wafer
and subsequently fabricate into Ga$_2$O$_3$ power MOSFETs. This LT approach is
essential to preserve the structural integrity of the composite wafer. These
LT-grown epitaxial Ga$_2$O$_3$ MOSFETs deliver high thermal performance (56%
reduction in channel temperature), high voltage blocking capabilities up to
2.45 kV, and power figures of merit of $\sim$ 300 MW/cm$^2$, which is a record
high for any heterogeneously integrated Ga$_2$O$_3$ devices reported to date.
This work is the first realization of multi-kilovolt homoepitaxial Ga$_2$O$_3$
power MOSFETs fabricated on a composite substrate with high heat transfer
performance which delivers state-of-the-art power density values while running
much cooler than those on native substrates. Thermal characterization and
modeling results reveal that a Ga$_2$O$_3$/diamond composite wafer with a
reduced Ga$_2$O$_3$ thickness ($\sim$ 1 $\mu$m) and thinner bonding interlayer
($<$ 10 nm) can reduce the device thermal impedance to a level lower than
today's GaN-on-SiC power switches.
