Although Pd/Ti/Pd/Au contacts are similar to their Pt/Ti/Pt/Au counterparts in providing low specific contact resistance, ρc, the former exhibits long-term thermal stability. Their projected mean times to 50% increase in ρc(μ50) at 150 °C to p+-GaAs (≥3.43×1015 h) are higher than those of the latter by over five orders of magnitude. Contacts to p+-In0.53Ga0.47As are not as thermally stable, with a much lower albeit respectable μ50 at 150 °C of ≥2.25×105 h. Contacts with an interfacial Pd layer provide ρc’s that are at least two times lower than those without, and the presence of an oxide layer (GaxTiyOz) at the Ti/GaAs interface is identified as a possible cause. Pd–Ga–As phases are formed at the Pd/GaAs interface, being As-rich (PdxGayAs) initially and convert to Ga-rich phases (PduGavAs) upon a high temperature anneal and the eventual composition depends on the evaporated interfacial Pd thickness and annealing conditions. This could probably explain the existence of an optimum interfacial Pd layer thickness of 100 Å for achieving the lowest ρc. The Ga-rich PduGavAs phases formed are inferred to cause the liberation of As atoms from the GaAs lattice, thus enabling them to diffuse out to the Ti and react to form TixAsy phases that bind the As from further out-diffusion. This has in turn led to the accumulation of As at the Pd/Ti interface. © 2000 American Institute of Physics.