We report on the optical properties of InAs/AlSb/GaSb based Type-II superlattice N-structure with p on n configuration. The detector structure is designed to operate in the mid wavelength infrared range with 50% cut-off wavelength of 4.88 mu m at 79 K. Electronic properties of N-structure such as heavy hole-light hole splitting energies are optimized by a first principles approach taking into account InAlAs interface bonding between InAs/AlSb layers. A responsivity of 1.13 A/Wat 3.2 mu m was measured under zero bias, with a corresponding quantum efficiency of 44%. An analytical model was developed in order to identify the contribution of quantum efficiency (QE) constituents. The overall QE contains a 56% contribution from the quasi-neutral p-region, 36% from the quasi-neutral n-region and 8% from the depletion region.