Failure Modes in GaAs Power FETs: Ohmic Contact Electromigration and Formation of Refractory Oxides

Abstract

Failure modes have been identified for two commercially available microwave power GaAs FETs constructed with aluminum gates. MTTF data for Al gate power FETs and Al gate in combination with refractory link power FETs-indicates the presence of a failure mode with a well defined activation energy. The first set of power FETs (Set A) use AuGe/Au source and drain contacts and Al gates. The second set use an Al-TiPt gate with AuGe-TiPt-Au source, drain contacts. The devices from set A were tested at 200°C under rf drive. An MTTF of 2200 hrs was achieved and source-drain electromigration was identified as the primary failure mode. This newly identified failure mode for ohmic contacts has been confirmed by Auger electron spectroscopy, and the SEM. Gallium has been shown to outdiffuse in the contact system with subsequent Ga whisker growth and gate void formation caused by the AuAl couple at the gate. Sputter AES profiles indicated that interdiffusion in the source and drain contacts extends into the active channel region under the bias-stress tests. The analyzed devices from the second set of power FETs (Al-TiPt gate with AuGe-TiPt-Au ohmic contacts) can be separated into four types: A, B, C, D. Type A failed catastrophically, type B failed non-catastrophically type C was annealed in nitrogen at 200-210°C for 150 hours and type D were untested devices which exhibited a high forward gate resistance.