A Au (1000 Å)/Ge (100 Å)/Pd (100 Å) contact scheme has been investigated to form low resistance Ohmic contact ton‐Al0.5In0.5P (Eg=2.3 eV) with a minimum contact resistivity of about 1×10−6Ω cm2. The surface morphology of this contact remained smooth after annealing at 425 °C for 1 min. Front side secondary ion mass spectrometry depth profiles of this contact structure under different annealing conditions were performed. It is found that the outdiffusion of indium due to the reactions between the metallization and the Al0.5In0.5P substrate in conjunction with the indiffusion of Ge into the substrate is responsible for the Ohmic contact formation. The germanide formation is believed to be responsible for the smooth surface morphology. The contact resistivity of this contact remained ∼2×10−6Ω cm2after aging at 350 °C for 31 h.