A Au (1000 Angstrom)/Ge (100 Angstrom)/Pd (100 Angstrom) contact scheme has been investigated to form low resistance Ohmic contact to n-Al0.5In0.5P (E(g)=2.3 eV) with a minimum contact resistivity of about 1x10(-6) Omega cm(2). The surface morphology of this contact remained smooth after annealing at 425 degrees 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 similar to 2x10(-6) Omega cm(2) after aging at 350 degrees C for 31 h.