This article presents the design and fabrication of a 6 W X-band hybrid Class-J power amplifier (PA) based on a bare die GaN on SiC HEMT by accurate implementing the transistor nonlinear capacitor effects. The transistor input capacitor is precisely modelled and its nonlinearity effects on Class-J performance is studied for the first time. It is shown that the harmonic generation property of the nonlinear input capacitor, especially at the second harmonic, can be of benefit to shape the transistor gate voltage as a quasi-half wave sinusoidal waveform and consequently, it can improve the power added efficiency (PAE). A complete 3D thermal model of the power transistor is developed using ANSYS software and it is calibrated based on the thermal measured data. The PA achieves 13 dB average power gain over the frequency range of 8.8-9.6 GHz. The drain efficiency and PAE are about 67% and 58% at 9.2 GHz, respectively.

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