  The self-heating effect is a severe issue for high-power semiconductor
devices, which degrades the electron mobility and saturation velocity, and also
affects the device reliability. On applying an ultrafast and high-resolution
thermoreflectance imaging technique, the direct self-heating effect and surface
temperature increase phenomenon are observed on novel top-gate \b{eta}-Ga2O3 on
insulator field-effect transistors. Here, we demonstrate that by utilizing a
higher thermal conductivity sapphire substrate rather than a SiO2/Si substrate,
the temperature rise above room temperature of \b{eta}-Ga2O3 on the insulator
field-effect transistor can be reduced by a factor of 3 and thereby the
self-heating effect is significantly reduced. Both thermoreflectance
characterization and simulation verify that the thermal resistance on the
sapphire substrate is less than 1/3 of that on the SiO2/Si substrate.
Therefore, maximum drain current density of 535 mA/mm is achieved on the
sapphire substrate, which is 70% higher than that on the SiO2/Si substrate due
to reduced self-heating. Integration of \b{eta}-Ga2O3 channel on a higher
thermal conductivity substrate opens a new route to address the low thermal
conductivity issue of \b{eta}-Ga2O3 for power electronics applications.
