  ${\beta}$-Ga${_2}$O${_3}$ based semiconductor devices are expected to have
significantly improved high-power and high-temperature performance due to its
ultra-wide bandgap of close to 5 eV. However, the high-temperature operation of
these ultra-wide-bandgap devices is usually limited by the relatively low 1-2
eV built-in potential at the Schottky barrier with most high-work-function
metals. Here, we report heterojunction p-NiO/n-${\beta}$-Ga${_2}$O${_3}$ diodes
fabrication and optimization for high-temperature device applications,
demonstrating a current rectification ratio of more than 10${^6}$ at
410{\deg}C. The NiO heterojunction diode can achieve higher turn-on voltage and
lower reverse leakage current compared to the Ni-based Schottky diode
fabricated on the same single crystal ${\beta}$-Ga${_2}$O${_3}$ substrate,
despite charge transport dominated by interfacial recombination. Electrical
characterization and device modeling show that these advantages are due to a
higher built-in potential and additional band offset. These results suggest
that heterojunction p-n diodes based on ${\beta}$-Ga${_2}$O${_3}$ can
significantly improve high-temperature electronic device and sensor
performance.
