A Pt/Ge/Au contact of the structure: 〈n- GaAs〉/Pt(17 nm)/Ge(25 nm)/Au(43 nm), overlaid with a Ta-Si-N barrier layer and a Au metallization layer has a contact resistivity, ρc, of 3.7×10-6 Ωcm2 after annealing at 450 °C for 15 min. After aging at 450 °C for 60 h, ρc slightly degrades to 5.5×10-6 Ωcm2 while the surface keeps smooth. When alloyed at 550 °C for 15 min, ρc is 1.8×10-6 Ωcm2 and stays about the same value after annealing at 550 °C for 1 h. Without the Ta-Si-N barrier and the Au overlayer, the Pt/Ge/Au contact alone is also ohmic after annealing at 450 °C for 15 min but with a ρc of approximately 10-5 Ωcm2 while the surface morphology deteriorates significantly after aging at 450 °C for 20 h. The thermal reactions of this Pt/Ge/Au contact on GaAs, with or without a Ta-Si-N barrier layer, are investigated by backscattering spectrometry, x-ray diffraction, and transmission electron microscopy in conjunction with energy dispersive analyses of x-rays. For all samples, the main reaction products after annealing at 450 °C for 15 min are Au7Ga2 and PtGe:As, a PtGe phase that also contains arsenic. The product phases are randomly distributed within a laterally uniform reacted layer when the Pt/Ge/Au contact is covered by a Ta-Si-N layer. Without the Ta-Si-N barrier layer, a small arsenic loss and a Ga-rich phase (probably Ga-oxides) on the contact surface are observed after annealing at 450 °C. In this case, the surface and contact-semiconductor interface are more faceted than with a Ta-Si-N barrier layer.