Novel surface treatments for an InAs0.91Sb0.09 p-n junction grown on GaSb by MOCVD are proposed and demonstrated. Exposure of the p-n junction to argon ion plasma followed by etching with tartaric acid reduces the dark currents by five orders of magnitude. An additional reduction by two orders of magnitude in the dark current is achieved by illuminating the sample with white light. A mid-wavelength infrared photodetector based on the so treated p-n junction exhibits a high zero-bias resistance-area product (R 0A) of 2.7 × 107 Ω cm2 and an internal quantum efficiency of 70% at 77 K. The measured BLIP temperature is 170 K and a high detectivity value (D* BLIP) of 4.03 × 1011 cm Hz1/2 W-1 is observed at 3.7 νm. The surface treatment results indicate that the device performance is limited by bulk material properties.