A comprehensive class A to B power and load-pull characterization of GaN HEMTs on SiC and sapphire substrates

An extensive power characterization of devices fabricated on GaN grown on SiC and sapphire substrates has been carried out, including power sweep and load-pull measurements in different bias conditions from class A to class B. An active load-pull bench optimized for high voltage and high power measurements allows to extend the load-pull characterization to the whole Smith chart, and to localize the optimum load conditions even for devices with almost reactive optimum terminations. The characterization procedure allows to verify scaling rules and the effects of defects and thermal degradation on the device power performances. The results of the SiC and sapphire-based devices show that, on one side, SiC-based devices exhibit state-of-the-art performances in Class A, and, on the other side, low-cost sapphire-based devices, when biased in high efficiency classes, can be viable candidates for medium power applications, despite the higher thermal resistivity of sapphire compared with the one of SiC