Abstract: In an infrared photodetection apparatus a photodetector diode is used which comprises a heterojunction of two epitaxial layers of differing compositions of a ternary III-V semiconductive alloy, such that the outer layer will serve as a radiation-admitting window as well as physical protection for the underlying absorbing layer in the so called direct photodetector diode configuration. The ternary alloy illustratively includes two metallic group III elements such as indium and gallium; but the principle can be extended to ternary alloys including two group V elements, such as arsenic and antimony. Further, quaternary alloys of III-V elements can be employed. The absorbing layer is selected to be substantially intrinsic. The latter is the case for an N-type layer of In.sub.x Ga.sub.(1.sub.-x) As. Matching of this absorbing layer to a gallium arsenide substrate is achieved by a plurality of step-graded composition layers of indium gallium arsenide.