This brief presents an active second-harmonic injection technique to improve the efficiency and bandwidth for high-efficiency power amplifiers (PAs). An optimum third-harmonic termination condition was examined for higher efficiency after the second-harmonic injection using a multiharmonic load-pull simulation. It was determined that the optimum third-harmonic termination is the same as that of the inverse class-F PA. Based on this result, a high-efficiency PA with an optimized third-harmonic termination for the second-harmonic injection was designed for a center frequency of 1 GHz as a main amplifier. The overall system requires an auxiliary second-harmonic amplifier and a diplexer between the main and auxiliary PAs. The PA with an optimized third-harmonic termination for the second-harmonic injection was implemented using a 10-W GaN high-electron-mobility transistor for both the main and auxiliary power stages. Compared with the PA without second-harmonic injection, the bandwidth with a power-added efficiency of more than 80% is extended from 60 (960-1020 MHz) to 180 MHz (880-1060 MHz) after the second-harmonic injection.