AlGaN/GaN HEMTs are promising candidates for high frequency applications with high power and low noise. While switching applications demand normally-off operation, conventional HEMTs are normally-on. To achieve normally-off HEMTs, several structures have been proposed. One of the major normally-off HEMTs uses fluorine implantation in the AlGaN layer. We suggest in this work the implantation of fluorine ions under the AlGaN/GaN interface only below the gate electrode rather than implanting in the AlGaN layer. Simulation results show that the proposed method is capable of achieving normally-off operation and is more effective when it comes to the fluorine concentration required to obtain a desired threshold voltage