Increased RF-Losses at the GaN/Si Interface after Eutectic Die Attach

The influence of eutectic die attach on RF-substrate losses of AlGaN/GaN HEMTs grown on high-resistivity silicon substrates has been studied. A severe degradation in load pull efficiency is observed after die attach. Analyzing this degradation for dies attached to flanges with differing thermal expansion coefficient shows a linear relationship between substrate strain and load pull efficiency. Loss measurements of coplanar waveguides (CPWs) show increasing substrate losses with increasing strain, while the transistor on-resistance remains unaffected. The degradation in load pull efficiency is, therefore, attributed to increased substrate losses. By evaluating the bias dependence of low frequency capacitance measurements together with the bias dependence of CPW characteristics, we show that the increase in substrate loss can be explained by the formation of interface charge at the GaN/Si interface. To the best of our knowledge, this is the first time that GaN transistor efficiency degradation has been shown to correlate with transmission line losses and changes at the GaN/Si interface after eutectic die attach.