Type-II strained layer superlattices (SLSs) offer a broad range of design degrees of freedom to help optimize their properties as absorber layers of infrared photon detectors. We theoretically examine a new class of mid-wavelength infrared (2-5 μm bandpass) Type-II structures with two-layer InGaSb/InPSb and four-layer InAs/GaSb/InAs/InPSb SLS periods. Phosphorous-containing SLSs are a promising approach to improving infrared photon detector performance due to providing a new set of material properties, including favorable valence band offsets. P-based SLSs of four-layer type InAs/GaSb/InAs/InPSb were found to be among the best 5-μm gap SLSs that we have modeled. Among the studied designs, the lowest dark current in an ideal structure is predicted for a four-layer 23.6 Å InAs/20 Å GaSb/23.6 Å InAs/60 Å InP 0.62 Sb 0.38 SLS. Its predicted ideal dark current is about 35 times lower than an n-type HgCdTe-based photodiode absorber and six times lower than a p-type HgCdTe one for the same bandgap, temperature, and dopant concentration. We also discuss a defect mitigation strategy that involves positioning the SLS gap in an energy range that avoids defect levels and show how this applies to the aforementioned P-containing SLS.