Simulation and Optimization of Gate Temperatures in GaN-on-SiC Monolithic Microwave Integrated Circuits

This paper presents 3-D thermal simulation studies of GaN-on-SiC monolithic microwave integrated circuits (MMICs) containing multifinger micrometer-scale high electron mobility transistors (HEMTs). The heat spreading effect of HEMT source, gate, and drain metallizations on peak structure temperatures is examined. The impacts of a realistic die attach material and rear-of-die heat transfer coefficient on structure temperatures, and in particular on temperature nonuniformity, are examined. Variable gate finger spacing, in which the gate spatial positions are described by polynomials as a function of gate number, is investigated as a means for optimizing the temperature uniformity from gate-to-gate. A thermal simulation code with a parametric MMIC geometry-based mesh generator and a deformable mesh consistent with sequential movement of gate finger positions during optimization is employed for all of the studies. The code is multiscale with a sufficient resolution range to handle a multifinger HEMT structure while also including the MMIC die, die attach metallization, and a realistic heat transfer coefficient associated with microchannel coolers. A variable gate pitch geometry based on an optimized cubic polynomial demonstrates considerable advantage in temperature uniformity.