One of the research hotspots for Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistors(SiC MOSFETs) is how to reduce the High-Frequency Figure of Merit(HF-FOM). This paper presents a theoretical derivation combined with Technology Computer Aided Design(TCAD) simulation technology to analyze the influence of field plate isolation dielectric materials on device performance. Based on this analysis, we propose a Split-Gate MOSFET(SG MOSFET) structure named HK(High-K)-MOSFET that connects the source field plate with a High-K field plate isolation dielectric. Compared to the HK MOSFET(SiO₂) using silicon oxide as the field plate dielectric and the conventional Split Gate structure(SG MOSFET), the gate-drain capacitance(C_GD) of the HK-MOSFET(HfO₂) using hafnium oxide as the dielectric is reduced by 34.0% and 71.7%, respectively, while the gate-drain charge(Q_GD) is reduced by 24.5% and 58.9%, respectively. The (HF-FOM)¹(C_GD×R_onsp) and (HF-FOM)²(Q_GD×R_onsp) decrease by 71.4% and 60.1% compared to those of the SG-MOSFET. The results indicate that utilizing High-K dielectrics is an effective approach for designing fast switching SiC MOSFET devices.