In this work a W-Band power amplifier is presented in pmb 0.1 μm GaAs pHEMT. The amplifier is composed of pmb1 × 4 and pmb1 × 8 driving stages for high gain and of three successive pmb 1 × 8 power stages for high power. The stages were designed using a developed generic unconditionally stable unit cell. The output combiner was designed very efficiently with only 0.75 dB insertion loss to present each active device with mathbf Z opt. The amplifier has a small-signal gain above 25 dB and more than 26 dBm P sat over 93-102 GHz. Due to the chosen topology, the amplifier presents a rapid saturation achieving almost P sat after 6 dB compression. The PA demonstrates a peak output power of 27.3 dBm between 96-98 GHz. The measured PAE is around 12% over 93-102 GHz. It is the first power amplifier in GaAs demonstrating 0.5W of output power above 90GHz. The chip size is pmb 3 × 3 mm 2 including pads.