In order to optimize the surface electric field of the traditional AlGaN/GaN high electron mobility transistor and improve the breakdown voltage and reliability, a new AlGaN/GaN high electron mobility transistor is proposed with the partial fixed positive charges in the Si3N4 passivation layer in this paper. The partial fixed positive charges of the Si3N4 passivation layer do not affect the polarization effect of the AlGaN/GaN heterojunction. The surface electric field tends to the uniform distribution due to the new electric field peak formed by the partial fixed positive charges, which modulates the surface electric field by applying the electric field modulation effect. The high electric fields near the gate and drain electrode decrease due to the new electric field peak. The breakdown voltage is improved from the 296V for the traditional structure to the 650V for the new structure proposed. The reliability of the device is improved due to the uniform surface electric field. The effect of the electric field modulation is explained by the horizontal and vertical electric field distribution between the Si3N4 and AlGaN interface, which provides a scientific basis for designing the new structure with the partial fixed positive charges in the Si3N4 layer. Because of the fixed positive charge compensation, the two-dimensional electron gas concentration increases, and the on-resistance decreases. So, the output current of the new structure increases compared with that of the traditional AlGaN/GaN High Electron Mobility Transistor.