Gate metallization "Sinking" into the active channel in Ti/W/Au metallized power MESFET\´s

Commercial power MESFET´s with Ti/W/Au gate metallization show a failure mode consisting of a decrease in Idssand Vpand an increase in R0. The failure mechanism was investigated by electrical and structural analyses with SEM, microprobe, and Auger spectrometry, and turns out to be a thermally activated Au-GaAs interdiffusion leading to encroachment of the gate metal on the channel. This may be common to all Au-metallized MESFET´s and can lead to burn-out of devices.