As was shown in [170,172], even for moderate RF frequencies, GaAs HEMTs, and pseudomorphic AlGaAs/InGaAs HEMTs (PHEMTs) on GaAs substrates offer the highest PAE, output power and gain on the FET side in this frequency range. Extremely high-power amplifiers at 2 GHz using three terminal devices with saturated output power = 200 W  have been realized. By introducing a fourth terminal output powers up to = 230 W  have been achieved. In the X-band (8-12 GHz) amplifiers up to = 39 dBm have been demonstrated, e.g. by TNO in . Combining these circuits in modules up to 70 W of output power have been realized .
In the K-band (18-26.5 GHz) PAE is a major concern especially for satellite communication. A PAE of 68% was demonstrated at 18 GHz using PHEMTs , where also a compilation of data on K-band power amplifiers is given.
High-power amplifiers have been developed for radar applications at 35 GHz to replace traveling wave tube (TWT) amplifiers. Recently, however, the Ka-band (26.5-40 GHz) has been faced with growing attention due to the interest in high bandwidth, high-speed digital data transmission applications such as Local Multipoint Distribution Services (LMDS) or Multichannel-Multipoint Distribution Services (MMDS). The highest overall output power for a Ka-band high-power amplifier on a single chip was realized by both  and  with = 5 W at 27.5 GHz to 29 GHz and = 4 W for = 29 GHz and 31 GHz, respectively. Very recently, = 6 W at 30 GHz on a single chip have been published in . LMDS chip solutions for mixers have been proposed, see e.g. [27,301]. An overview for LMDS modules is given in . Module solutions up to = 6 W have been demonstrated by TRW in . A space qualified process for this frequency has been reported by UMS in , or in a more complete description in . Low noise amplifiers at 38 GHz were demonstrated by Ali et al. in .