Abstract: The invention relates to a method of treating a substrate of gallium arsenide by a double ion implantation. A first implantation of silicon ions (Si.sup.+) is carried out on the entire surface of the substrate, and a second implantation of oxygen ions (O.sup.+) is carried out in regions intended to become isolated regions. A thermal annealing treatment, preferably under encapsulation, follows these ion implantations. These implantations are carried out in order to obtain at the surface of the substrate regions of n-conductivity type isolating regions separated from each other for subsequent manufacture of semiconductor devices. The invention also relates to a gallium arsenide substrate thus treated and to a semiconductor device obtained by the technique of two ion implantations.