The theoretical analysis of a single-ended Class E/F n mode with explicit derivation of the idealized optimum voltage and current waveforms and load-network parameters with their verification by time- and frequency-domain simulations for a particular case of Class E/F3 mode with a 50% duty cycle are presented. The ideal collector voltage and current waveforms for driving signals with 50% duty cycles demonstrate a possibility of 100% efficiency without overlapping between each other. Two examples of the Class E/F3 GaN HEMT power amplifiers, one with lumped elements at operating frequency of 430 MHz and the other with transmission-line elements at operating frequency of 2.14 GHz, are described and analyzed based on the simulation results. The test board with implemented transmission-line Class E/F3 GaN HEMT power amplifier has been measured and high-performance results with the output power of 40 dBm, drain efficiency of 76%, power-added efficiency of 73.1%, and power gain of 14.3 dB were achieved at operating frequency of 2.14 GHz.