  In this letter, fin-shape tri-gate $\beta$-Ga$_{2}$O$_{3}$ lateral MESFETs
are demonstrated with a high power figure of merit of 0.95 GW/cm$^{2}$ - a
record high for any $\beta$-Ga$_{2}$O$_{3}$ transistor to date. A
low-temperature undoped buffer-channel stack design is developed which
demonstrates record high Hall and drift electron mobilities in doped
$\beta$-Ga$_{2}$O$_{3}$ channels allowing for low ON resistances R$_{ON}$ in
$\beta$-Ga$_{2}$O$_{3}$ MESFETs. Fin-widths (W$_{fin}$) were 1.2-1.5 $\mu$m and
there were 25 fins (N$_{fin}$) per device with a trench depth of $\sim$1$\mu$m.
A $\beta$-Ga$_2$O$_3$ MESFET with a source-drain length of 6.4 $\mu$m exhibits
a high ON current (187 mA/mm), low R$_{ON}$ (20.5 $\Omega$.mm) and a high
average breakdown field (4.2 MV/cm). All devices show very low reverse leakage
until catastrophic breakdown for breakdown voltages scaled from 1.1kV to
$\sim$3kV. This work demonstrates the potential of channel engineering in
improving $\beta$-Ga$_{2}$O$_{3}$ device performance toward lower conduction
losses for low-to-medium voltage applications.
