NiGeAu and PdGeTiPt ohmic contacts ton-GaAs and TiPd and PdGeTiPt ohmic contacts top+-GaAs are examined by comparing their contact resistances, chemical intermixing as determined by Auger electron microscopy, interface structure as determined by transmission electron microscopy, and surface roughness as determined by surface profiling all measured as a function of annealing time and temperature. Then-PdGeTiPt contact annealed for short times, ⩽15 s, and at low temperatures, ⩽395 °C, was superior to the NiGeAu contact because it had a comparable contact resistance, less interface mixing, better lateral homogeneity, and a smoother surface. However, its contact resistance increased substantially with the annealing time and temperature, whereas the NiGeAu contact was relatively unaffected. For all annealing times and temperatures except the one at 550 °C, the TiPd contact top+GaAs was superior as it had a lower contact resistance and a comparable amount of interface intermixing, lateral homogeneity, and surface roughness. However, it had a complete chemical breakdown at 550 °C, whereas the PdGeTiPt contact resistance remained relatively stable.