  Nano-membrane tri-gate beta-gallium oxide (\b{eta}-Ga2O3) field-effect
transistors (FETs) on SiO2/Si substrate fabricated via exfoliation have been
demonstrated for the first time. By employing electron beam lithography, the
minimum-sized features can be defined with a 50 nm fin structure. For
high-quality interface between \b{eta}-Ga2O3 and gate dielectric, atomic
layer-deposited 15-nm-thick aluminum oxide (Al2O3) was utilized with
Tri-methyl-aluminum (TMA) self-cleaning surface treatment. The fabricated
devices demonstrate extremely low subthreshold slope (SS) of 61 mV/dec, high
drain current (IDS) ON/OFF ratio of 1.5 X 109, and negligible transfer
characteristic hysteresis. We also experimentally demonstrated robustness of
these devices with current-voltage (I-V) characteristics measured at
temperatures up to 400 {\deg}C.
