Reliability Investigations of GaAs Power FETs with Aluminium Gate Metallisation

Accelerated life tests on GaAs power FETs have revealed two failure mechanisms. The first, associated with Au/Al interaction at the bonding pads, is gate void formation for which the estimated activation energy of 2eV in air ambient gives an extrapolated MTTF at 70°C channel temperature in excess of 108 hours. Devices with Au bond wires to the gate bonding pads show no void formation but failure is associated with excessive drain metal migration. Estimated MTTF at 70°C channel temperature is comparable to the gate failure mode. A detailed comparison of the two failure modes and conclusions as to which dominates under normal operating conditions await the outcome of constant stress tests on devices with both wire bond configurations.