High Electron Mobility Transistors (HEMTs) based on GaN are attractive for high-speed and high-voltage applications. The performance advantages of the GaN HEMTs rely on the high breakdown field of the GaN material and the high electron mobility of the 2-dimesional electron gas (2DEG) in the AlGaN/GaN heterojunction [1, 2]. In order to take full advantage of the excellent material properties, the shape of the electric-field distribution in the GaN HEMTs must be carefully optimized to operate the device at its highest switching speed while handling a large voltage swing. Without proper field-shaping, a high electric-field can cause electron injection into traps, hence degrading the output current and on-resistance during switching operation. This phenomenon is often referred to as dynamic on-resistance (Ron dynamic) degradation, current collapse or DC-RF dispersion. As an effective approach of shaping the electric-field, the use of field-plates in GaN HEMTs has received extensive studies . For microwave applications, a V-shaped gate with integrated sloped field-plate was used to control the electric-field with minimal added capacitance associated with the field-plate [5, 6]. For high-voltage applications, a multiple field-plates structure was used to scale up the operating voltage [2, 4]. In this paper, we report a sloped field-plate approach for high-voltage applications.