A new process approach to realize a slant field plate;a field plate (FP) gradually separated from the semiconductor surface from the gate edge toward the drain;is reported. A multistep SiCN dielectric film is used to make a sloped sidewall in the dielectric film consisting of a slant FP. The sidewall shape is controlled by the number of steps in SiCN. AlGaN/GaN high-electron-mobility transistors (HEMTs) with slant and conventional FPs are prepared using this technique. The advantage of slant FPs over conventional FPs is experimentally confirmed as a result of reduced current collapse, higher current gain cutoff frequency, and higher off-state breakdown voltage.