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A series of RF operating life tests has been performed on microwave transistors with different metallization systems to evaluate failure mechanisms and compare the lifetimes expected from the devices. An initial series of high temperature CW tests indicated that aluminum metallized devices fail after 2000h at 225°C, primarily due to voids in the emitter fingers caused by electromigration. Gold metallized transistors were found to be capable of long life at very high temperatures (2000 h at 340°C). A series of long-term pulsed life tests was performed at near operational temperatures on the same devices. The results confirmed the lifetime prediction for the aluminum devices based on the CW tests, but the failure mechanisms were somewhat different. The devices failed due to an emitter-base shunting mechanism, another manifestation of electromigration. The refractory metal/gold metallized devices failed because of metal peeling, a process related problem.