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We have successfully developed high-efficiency and high-power microwave amplifiers using a GaN field-effect transistor (FET) on a low-resistivity (LR) Si substrate for the first time. By introducing the LR Si substrate whose resistivity is less sensitive over temperature, the efficiency characteristics in high-temperature operation were significantly improved. In order to overcome the RF loss increase due to the utilization of the LR Si substrate, we have optimized the device structure of the FET by using an RC loss model accounting for drain-to-source capacitance and substrate resistance. High-efficiency characteristics were realized by optimizing the buffer structure and electrode structure. Furthermore, we have investigated efficiency improvement by second harmonic termination. The developed single-ended amplifier realized the saturation output power of 54 dBm, the linear gain (GL) of 19 dB, and the high efficiency of 33.5% at 8-dB back-off output power level under a WCDMA signal condition of 2.14 GHz at 50-V operation. The inverted Doherty amplifier using a pair of the 250-W GaN FETs delivered the of 57.3 dBm with the GL of 15.5 dB under a pulsed continuous wave signal condition of 2.14 GHz, and demonstrated the digital pre-distortion linearization characteristics with the high efficiency of 48% and the adjacent channel leakage power ratio of 55 dBc at 50 dBm. These results are comparable to the best performance among the ever reported GaN on SiC FET high-power amplifiers.Index Terms-GaN field-effect transistor (FET), high efficiency, high power, high temperature, inverted Doherty amplifier, microwave amplifiers, resistivity, second harmonic, Si substrate.

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This paper reports an Lband power AIGaAdGaAs heterostructure FET with a field-modulating plate (FP-HFET), which accomplished lOOW output power with a high power density of 1.16W/mm at a drain bias voltage of 30V. The developed FP-€WET is promising for achieving improved performance and reduced size of digital cellular base station systems.