  Field-plated (FP) depletion-mode MOVPE-grown $\beta$-Ga$_2$O$_3$ lateral
MESFETs are realized with superior reverse breakdown voltages and ON currents.
A sandwiched SiN$_x$ dielectric field plate design was utilized that prevents
etching-related damage in the active region and a deep mesa-etching was used to
reduce reverse leakage. The device with L$_{GD}$ = 34.5 $\mu$m exhibits an ON
current (I$_{DMAX}$) of 56 mA/mm, a high I$_{ON}$/I$_{OFF}$ ratio $>$ 10$^8$
and a very low reverse leakage until catastrophic breakdown at $\sim$ 4.4kV.
The highest measurable V$_{BR}$ recorded was 4.57 kV (L$_{GD}$ = 44.5 $\mu$m).
An LFOM of 132 MW/cm$^2$ was calculated for a V$_{BR}$ of $\sim$ 4.4 kV. The
reported results are the first $>$ 4kV-class Ga$_2$O$_3$ transistors to surpass
the theoretical FOM of Silicon. These are also the highest I$_{DMAX}$ and
lowest R$_{ON}$ values achieved simultaneously for any $\beta$-Ga$_2$O$_3$
device with V$_{BR}$ $>$ 4kV to date. This work highlights that high breakdown
voltages (V$_{BR}$), high lateral figure of merit (LFOM) and high ON currents
can be achieved simultaneously in $\beta$-Ga$_2$O$_3$ lateral transistors.
