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Temperature rising which originates from self‐heating degrades the electrical characteristics, reliability, and lifetime of high‐power GaN‐HEMTs. In this article, a systematic analytical approach for thermal evaluation of microwave GaN‐HEMTs is constructed through combining and scrutinizing some of the basic static thermal analysis methods to provide a deeper insight into the process of the channel temperature rising and self‐heating with a much lower computational burden. The proposed systematic thermal analysis has been applied to two different assembly methods: conventional mounting and flip‐chip mounting. Although the mathematical and empirical equations used are simple enough to save time, the effects of several phenomena and different conditions on the channel temperature have been taken into account. These include heat crowding phenomenon, the effect of temperature‐dependent thermal conductivity of the transistor constituent materials, transistor geometry, die‐attach material properties, and bump dimensions. To validate the accuracy of the calculations, all the analytical analyses are followed by 3D thermal simulations in ANSYS software. The simulation results confirm the accuracy of the analytical calculations.

This paper presents an 8.8-9.8 GHz hybrid power amplifier (PA) designed with GaN HEMT bare die transistors. Generally, hybrid X-band high power amplifier (HPA) suffers from relatively low bandwidth. However, this paper introduces wideband high gain power amplifier, by using low loss Wilkinson combiner/divider, a modified form of taper-based divider, and wideband matching structures. The optimum values of all elements are determined by using Random and Gradient algorithms. The microstrip matching structures are realized on Rogers4003 and RT/druid5880 substrates. Large-signal simulation results show peak efficiency of 40%, an output power of 100 W and power gain of 50 dB at 9.5GHz. The input and output return losses are respectively less than -12 dB and -10 dB.