Heterosubstrates have been extensively studied as a method to improve the heat dissipation of Ga₂O₃ devices. In this simulation work, we propose a novel role for p-type available heterosubstrates, as a component of a reduced surface field (RESURF) structure in Ga₂O₃ lateral field-effect transistors (FETs). The RESURF structure can eliminate the electric field crowding and contribute to higher breakdown voltage. Using SiC as an example, the designing strategy for doping concentration and dimensions of the p-type region is systematically studied using TCAD modeling. To mimic realistic devices, the impacts of interface charge and binding interlayer at the Ga₂O₃ /SiC interface are also explored. Additionally, the feasibility of the RESURF structure for high-frequency switching operation is supported by the short time constant (∼0.5 ns) of charging/discharging the p-SiC depletion region. This study demonstrates the great potential of utilizing the electrical properties of heat-dissipating heterosubstrates to achieve a uniform electric field distribution in Ga₂O₃ FETs.