THERMALLY STABLE WTIAU NONALLOYED OHMIC CONTACTS ON IN0.5GA0.5AS FOR GAAS-ALGAAS HETEROJUNCTION BIPOLAR-TRANSISTOR APPLICATIONS

This paper demonstrates WTiAu as a thermally stable, low resistance, n on-alloyed, emitter ohmic contact for GaAs-AlGaAs heterojunction bipol ar transistor applications. The minimum W layer thickness required for low contact resistance and long-term thermal stability was obtained. A W layer 900 angstrom thick yielded the lowest contact resistance (R( c) = 0.045 OMEGA mm) with a high degree of uniformity after (1) a coll ector ohmic contact rapid thermal annealing cycle at 420-degrees-C and (2) 500 h at 250-degrees-C. Secondary ion mass spectroscopy results i ndicate that In outdiffusion contributes to the thermal instability wh en thinner W layers are used.