Failure Mechanisms in Pulsed RF Power Transistors

Reliability life tests of rf power transistors during pre-production evaluation of advanced design avionics distance measuring equipment (DME) have revealed two primary failure mechanisms affecting the transistors used in this 1 GHz pulse power application. The mechanisms are: 1. dissolution of silicon in aluminum, with subsequent hillock formation in the emitter metallization, and 2. aluminum grain swelling, both mechanisms resulting in base-emitter degradation. These results have been verified by SEN and electrical tests based on rf pulse power life tests on the DME equipment and from failures in field test environments. Several corrective measures have been implemented by semiconductor manufacturers as a consequence of aluminum electromigration failures in preliminary carrier wave (CW) life tests. Additional corrective measures on these devices were necessary to eliminate the failure mechanisms seen in the 1 GHz pulse power environment to which these transistors were subjected. A recent 5000 hour pulse power life test substantiates that degradation has not occurred in transistors incorporating the design corrections.