In this paper, a novel GaN MESFET structure with Double Extra Layers of Si3N4 (DEL-MESFET) under gate edges has been introduced in order to modify the carrier concentration and the electric field in the channel. The results show that the breakdown voltage (VBR) increases to 200 V in the proposed structure from 119.2 V in a conventional MESFET structure (C-MESFET). Furthermore, the maximum output power density (Pmax) of the proposed structure is 74% higher than that of the conventional one. By modifying the carrier's concentration, the gate-source and gate-drain capacitances decrease that lead to the improvement of the RF characteristics including ft and fmax from 27.5 and 99 GHz of the C-SOI MESFET to 30 and 105 GHz of the DEL-MESFET, respectively. Therefore, the results show that the proposed structure provides the excellent performance compared with the C-MESFET structure and can be taken into consideration for the future of the VLSI applications.