Abstract: Useful semiconductor materials in which a p-n junction may be formed are made from compositions of the quaternary Ga.sub.y In.sub.1-y As.sub.1-x P.sub.x compound alloy system having values of x and y greater than 0.005 and less than 0.995 or having a value of x equal to 1.0 and a value of y from 0.45 to 0.80. In making these semiconductor materials a layer of selected composition of this compound alloy system is epitaxially grown on a substrate having a lattice constant substantially equal to that of the selected composition by substantially following an isolattice constant contour to the region of selected composition in the alloy system. Injection electroluminescent diodes are fabricated from these semiconductor materials by forming a p-n junction in the layer of selected composition and by forming electrical terminals in contact with the substrate and the layer of selected composition on opposite sides of the p-n junction.