Thermal Model Extraction of GaN HEMTs for Large-Signal Modeling

Self-heating has a large effect on electrical performance of RF power devices. In the past, several methods were developed to estimate the average channel temperature of FETs. Some of these are based on approximate closed form expressions. These techniques give acceptable results under limited conditions and only for specific device layouts. In this proposed work, we present an accurate method for the extraction of the thermal profile of large-size AlGaN/GaN HEMTs using both Finite Element Method (FEM) simulation and measurement techniques. The thermal investigation of the complete structure of the device permits an accurate calculation of the distributed device temperature taking into account the temperature dependence of the thermal conductivities. This analysis also helps device designers tuning physical and geometrical parameters of the structure. The thermal resistances and the thermal time constants under each finger and of the whole FET structure are calculated from static and transient FEM thermal simulations, respectively. Alternatively, the thermal time constant is also determined from drain current transient measurements. Using this procedure, we obtained detailed thermal profile for AlGaN/GaN HEMT and implemented the resulting thermal sub-circuit in the large-signal model of GaN HEMTs. The simulated static and transient I(V) characteristics of the large-signal model are in good agreement with the measured data.