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A new GaAs pHEMT structure, which has achieved both high power density and high breakdown voltage for millimeter-wave amplifiers, is presented. According to the nonlinear drain resistance model previously proposed, we have adopted a "Stepped Recess" structure, which includes a GaAs buried layer 150 nm thick to reduce the nonlinear drain resistance and a recessed area at the gate side to control the gate leakage current. Consequently, even at the 2.0 mum width of the gate-to-drain separation, the developed pHEMT has showed a high power density of 0.65 W/mm in the Ka band, and high on-state breakdown voltage of over 30 V at once. In addition, the proposed nonlinear drain resistance model effectively explains this power performance.