The electrical properties and microstructure of InAs ohmic contacts ton‐type GaAs, prepared by sputter‐depositing a single target, were studied by measuring the contact resistance (Rc) by the transmission line method and analyzing the interfacial structure by x‐ray diffraction and cross‐sectional transmission electron microscopy. Current‐voltage measurement of an as‐deposited InAs/W contact showed Schottky behavior, where the W layer was used as a cap layer. The InAs layer had an amorphous structure and a uniform oxide layer was observed at the InAs/GaAs interface. Even after annealing at 800 °C, ohmic behavior was not obtained in this contact because the intervening oxide layer prevented the InAs and GaAs interaction. By adding Ni to the InAs/W contacts (where Ni was deposited by an evaporation method), the interaction between the InAs and the GaAs was enhanced. Nickel interacted with As in the InAs layer and formed NiAs phases after annealing at temperature above 600 °C. The excess In in the InAs layer reacted with the GaAs substrate, forming InxGa1−xAs phases which covered about 80% of the GaAs interface. TheRcvalues of ∼0.4 &OHgr; mm were obtained for InAs/Ni/W and Ni/InAs/Ni/W contacts at annealing temperatures in the range of 750–850 °C. These contacts contained only high melting point compounds and the contacts were stable during annealing at 400 °C for more than 100 h after ohmic contact formation.