High-Efficiency and High-Power Microwave Amplifier Using GaN-on-Si FET With Improved High-Temperature Operation Characteristics

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 (C_ds) 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 (P_sat) 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 P_sat 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.