Embodiments relate to photodetectors comprising: a substrate and a bulk-alloy infrared (IR) photo absorption layer disposed on the substrate to absorb photons in an infrared wavelength and having a graded section and an ungraded section. The photodetector comprises a unipolar barrier layer disposed on the bulk-alloy photo absorption layer. The graded section includes a graded alloy composition such that its energy bandgap is largest near the substrate and smallest near the unipolar barrier layer. The embodiments also relate to methods fabricating the photodetectors.