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In this paper, a AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (PHEMT) monolithic microwave integrated circuit (MMIC) power amplifier for Ka band applications is presented. PHEMT AlGaAs/InGaAs/GaAs epi-layers are grown by metal organic chemical vapor deposition (MOCVD) and 100 nm gate lengths is realized by double exposure e-beam lithography method. The device fabricated by the double exposure processes is presented maximum saturation current densities (IDS max) of 680 mA/mm and the peak extrinsic transconductance (gm max) of 485 mS/mm, respectively. In case of RF performance, current gain cutoff frequency (fT) of 56 GHz, maximum stable gain (fmax) of 84 GHz and yielding a power density of 10 W/mm at 30GHz are indicated. The three-stage MMIC power amplifier is designed to fully match the 50 Ω input and output impedances. With 6 V and -0.75 V DC bias, a 1 dB compression power (P1dB) of 29.5 dBm, a power added efficiency (PAE) of 31% are achieved from 28.5 to 31.5 GHz. The proposed Ka-band power amplifier is designed within a die size of about 3.3 × 1.9 mm2 on GaAs substrate.