A K-band high efficiency high output power frequency doubler in a 0.5-μm GaAs enhancement- and depletion-mode pseudomorphic high electron-mobility transistor (E/D-mode PHEMT) technology is presented in this paper. The doubler employs a configuration of a common-gate (CG) field effect transistor (FET) /common-source (CS) FET pair to enhance the second harmonic efficiently. Between 21 and 28 GHz, this doubler features a conversion gain of higher than -1.5 dB with an input power of 8 dBm. The maximum conversion gain is 1.9 dB at 26 GHz with an efficiency of up to 23.2% and a power-added efficiency (PAE) of 9.6 %. The maximum output 1-dB compression point (P1dB) is 10.4 dBm and the saturation output power (Psat) is higher than 11.4 dBm at 24 GHz. The overall chip size is 1×1 mm2. To the best of the author's knowledge, this work demonstrates the highest efficiency with high output power among all the fully integrated doublers covering the K-band without buffer amplifiers.