A mode-transferring technique for designing high-frequency high-efficiency broadband power amplifiers is presented in this paper. It is demonstrated, for the first time, that by properly tuning the second and third harmonic impedances, a PA can operate between inverse Class-F and Class-F modes within a 1.5 ∶ 1 bandwidth. A broadband PA is designed by employing this technique with a commercial GaN transistor. The multi-mode PA operation and broadband matching are realized simultaneously using a three-stage, transmission-line-implemented, low-pass network. Simulation and experimental results show that Class-F−1 and Class-F PA modes are successfully performed at 1.8 and 2.8 GHz, respectively, with ≥ 80% measured efficiency. In addition, an overall bandwidth of 1.3–3.3 GHz is achieved by the implemented PA with > 10-dB gain, 60%–85% efficiency, and 10-W output power throughout this band.