Thermal simulations of gallium nitride (GaN)-based microwave and power devices in steady and transient states are presented. The temperature distributions in steady states and temperature variations in transient states are simulated using the finite element method (FEM). Foster-type equivalent thermal circuits are extracted from the transient thermal responses using the FEM. When the number of RC parallel circuits is larger than the number of layers in the devices, the equivalent thermal

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... s enable the simulation in the time domain at switching frequencies on the order of MHz. Thermal simulation using those circuits for a switching operation shows that the temperature variations of the GaN-based microwave and power devices are 16.9°C and 1.7°C, respectively. This is due to the differences in the dissipated power density of the total area of the heat sources.