The winning power amplifier implemented with a gallium nitride (GaN) high electron mobility transistor (HEMT) and having power added efficiency (PAE) greater than 85%. It is shown that computer-aided design (CAD) simulation tools, accurate device models, and sensible design rule can produce first-pass power amplifier design success. To design a a power amplifier with highest efficiency at an output power above 5W and at a frequency above 1GHz, the amplifier was designed for an output frequency of 1.2 GHz, an output power of greater than 38 dBm, and an efficiency above 80%. The transistor chosen for this design was a 10-W RF power GaN HEMT and is available in a screw-down package and is not internally matched. It comes with an accurate nonlinear model, which is critical for the design of high efficiency power amplifiers. The efficiency generated by this method was 88% at 1.2 GHz with an output power just above 6W.