GaAs MMIC thermal modeling for channel temperatures in accelerated life test fixtures and microwave modules

Detailed thermal modeling of a gallium arsenide (GaAs) power amplifier monolithic microwave integrated circuit (MMIC) yields operating channel temperatures that are used to correlate reliability life test results. The model includes temperature dependent material properties, surface metallization layers, and volumetric heat generation in the depletion region directly beneath the channels. Also included are chip-to-substrate and substrate-to-housing interface thermal resistances. Model predictions which include the top surface metallization layers indicate the hottest channel is not always the center channel as simpler methods would predict but in a location with partially unplated metallization. The finite difference meshing scheme is first verified by comparison to a simplified geometry that may be characterized by an analytical solution program. After the channel temperatures are established over a range of temperatures, model verification is accomplished by infrared (IR) imaging. The necessity of coating the GaAs MMIC with a uniform emissivity material to obtain accurate IR imaging results is demonstrated. A final confirmation of the techniques is by photographs taken during failure analysis indicating device failures occurred at the location predicted by the thermal model.<>