Ohmic contacts to heavily C‐doped AlGaAs were made using PdGeTiPt that had specific contact resistancesRc, as low as 1.7×10−6&OHgr; cm2when annealed at 600 °C. The less heavily doped samples annealed at temperatures between 350 and 500 °C were non‐Ohmic, andRcdecreased with increasing annealing temperature between 500 and 600 °C. For the more heavily doped samples,Rcdecreased with increasing annealing temperature.Rcincreased for all samples at annealing temperatures above 600 °C.Rcrose quickly by 102when the samples were reannealed at 300 °C for 20 h, but remained unchanged with further reannealing for up to 100 h. This behavior is consistent with partial compensation generated by the rapid out‐diffusion of Ga at low annealing temperatures and the subsequent in‐diffusion of Ge into the Ga vacancies left behind. The lowerRcobtained with the 600 °C anneal can be explained by an increased As out‐diffusion and the subsequent in‐diffusion of Ge into the As vacancies at the higher annealing temperatures. Interfacial reactions and elemental diffusion of the contacts investigated via transmission electron microscopy and elemental depth profiles obtained by Auger electron spectroscopy are also consistent with this mode. ©1995 American Institute of Physics.