Failure mechanisms in gold and aluminum microwave power transistors

A series of step stress, accelerated life tests and failure analyses was carried out on microwave power transistors to determine the mechanisms by which they fail, and to obtain an estimate of their expected lifetimes in use. Both gold and aluminum metallized transistors were evaluated. These tests have revealed that the aluminum devices are susceptible to at least two failure mechanisms, emitter-base degradation and electromigration. Electromigration was also observed on the gold devices. The aluminum devices tested fail, typically, after 150 hours at 280 C and 700 hours at 250. If these data are extrapolated to a more normal junction temperature of 150 C, a median-time to failure of 500,000 hrs is obtained. The gold metallized devices have been life-tested at junction temperatures of 340 C for as long as 2000 hours without failing.