The authors describe a Pd/Ru/Au Ohmic contact with enhanced thermal stability over the more commonly used Pd/Pt/Au Ohmic contact for InAlSb/InAs high electron mobility transistors. Transmission electron microscopy shows that reaction between Pd and the semiconductor begins in samples even before they are annealed. Decreases in contact resistance are correlated with increasing reaction between Pd and the semiconductor for annealing and aging at 175-225 degrees C for 3 h or 1 week. Small voids form in severely aged samples but do not increase the contact resistance. The Ru diffusion barrier is never observed to react with any other materials in either the contact or the semiconductor, and Au remains isolated from the Pd-bearing reaction products and semiconductor. (C) 2009 American Vacuum Society. [DOI:10.1116/1.3204983]