In this paper, we demonstrate an nBn mid-wavelength infrared (MWIR) photodetector with a 100% cutoff wavelength over 4.0 µm based on a high-In composition InGaAsSb absorber on GaSb substrate. A thin AlGaAsSb layer is exploited as the barrier layer to suppress the generation-recombination (G-R) current. Both structural and optical properties of the grown InGaAsSb epilayer are investigated by high-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) measurement. Low dark current density down to 9.9 × 10−5 A/cm2 and 6.5 × 10−2 A/cm2 are obtained at −500 mV under 77 K and 300 K, respectively. Moreover, respective peak responsivity of 0.64 A/W and 0.10 A/W at 2.67 µm are achieved at −500 mV under 77 K and 160 K. These correspond to a peak specific detectivity of 1.06 × 1011 cm·Hz1/2/W and 4.23 × 109 cm·Hz1/2/W, respectively.