The Schottky barrier height and ideality factor of Cr-Ni-Co alloy Schottky contacts on a n GaAs substrate have been measured using a current voltage (I-V) technique after different rapid thermal annealings (RTA) for 20 s in a vacuum of almost-equal-to 10(-5) torr (temperature range from 299 to 900 K). The value of barrier height PHI(b) and ideality factor n range from 0. 57 eV at 1.14 (at room temperature) to 0.67 eV and 1.00 (at 673 K). Nonideal values of PHI(b) and n are attributed to the presence of an interfacial layer between the alloy and the semiconductor interface. However, the value of 1.00 for n at 673 K shows that annealing up to this temperature causes a disapperance of the I-V nonidealities. The fact that the barrier height increases and the ideality factor decreases to unity is ascribed to reduction of the interfacial oxide layer by metal Cr. On the other hand, the contact properties of the Schottky diodes deteriorated and became nearly ohmic when the annealing temperature reached 900 K for 20 s. This showed that there are not sufficient quantities of nickel in our Cr-Ni-Co alloy. The excellent saturation of the reverse bias curves which are due to the smoothness and quality of the metal/GaAs interface (even with annealing to a high temperature) are attributed to the existence of Co in the alloy.