A thin Silicon-On-Insulator (SOI) LDMOS with ultralow Specific On-Resistance (Ron,sp) is proposed, and the physical mechanism is investigated by Sentaurus. It features a FIN gate and an extended superjunction trench gate to obtain a Bulk Electron Accumulation (BEA) effect. The BEA consists of two p-regions and two integrated back-to-back diodes, then the gate potentialVGSis extended through the whole p-region. Additionally, the gate oxideWoxideis inserted between the extended superjunction trench gate and N-drift. In the on-state, the 3D electron channel is produced at the P-well by the FIN gate, and the high-density electron accumulation layer formed in the drift region surface provides an extremely low-resistance current path, which dramatically decreases theRon,spand eases the dependence ofRon,spon the drift doping concentration (Ndrift). In the off-state, the two p-regions and N-drift deplete from each other through the gate oxideWoxidelike the conventional SJ. Meanwhile, the Extended Drain (ED) increases the interface charge and reduces theRon,sp. The 3D simulation results show that theBVandRon,spare 314 V and 1.84 mΩ∙cm−2, respectively. Consequently, theFOMis high, reaching up to 53.49 MW/cm2, which breaks through the silicon limit of the RESURF.