This paper describes the design of a 28 GHz power amplifier (PA) in a commercial GaAs mHEMT technology using concepts that are typical for mm-wave CMOS design. Simulations show a 1dB output compression point of around 23 dBm with a 30% power-added efficiency (PAE) at 28 GHz, while providing a gain of 12 dB. Comparison with the performance of a similar 28 GHz fully-depleted Silicon-On-Insulator (FDSOI) PA shows an increase of the compression point with 10 dB while efficiency is comparable. The high compression point of this GaAsPA offers a margin for system optimization such as a reduction of the number of antennas for beamforming.