In this paper we report a theoretically simulated Hg1-xCd xTe based heterojunction medium wavelength infrared (MWIR) photodetector for operation at temperature 78K. The detector is designed to operate at strategic atmospheric window in the MWIR (3.8 μm) region with a high value of detectivity (>108 mHz1/2W-1) and quantum efficiency (- 60%). The device has been modeled using closed form analytical formula and is also simulated using the device simulation software ATLAS from SILVACO® international. The device has been theoretically characterized using the above models in respect of energy band diagram, electric field profile, current voltage characteristics, spectral response, quantum efficiency and detectivity. The detector is expected to find application in free space optical communication at 3.8 μm. © 2008 IEEE.