  This work presents the electrostatic analysis of a novel Ga$_2$O$_3$ vertical
Schottky diode with three different guard ring configurations to reduce the
peak electric field at the metal edges. Highly doped p-type GaN, p-type
nonpolar AlGaN and polarization doped graded p-AlGaN are simulated and analyzed
as the guard ring material, which forms a heterojunction with the Ga$_2$O$_3$
drift layer. Guard ring with non-polar graded p-AlGaN with a bandgap larger
than Ga$_2$O$_3$ is found to show the best performance in terms of screening
the electric field at the metal edges. The proposed guard ring configuration is
also compared with a reported Ga$_2$O$_3$ Schottky diode with no guard ring and
a structure with a high resistive Nitrogen-doped guard ring. The optimized
design is predicted to have breakdown voltage as high as 5.3 kV and a specific
on-resistance of 3.55 m$\Omega$-cm$^2$ which leads to an excellent power figure
of merit of 7.91 GW/cm$^2$.
