The fabrication and characterization of heterojunction phtodiodes for room temperature operation in the mid-infrared (2-5 µm) spectral range is described. Liquid phase epitaxy was employed to fabricate two different devices containing In0.97Ga0.03As and InAs0.89Sb0.11 active regions appropriate for phtodetection at 3.3 µm and 4.6 µm, corresponding to the absorption bands of methane and carbon monoxide. Basic detector characteristics have been measured and were found to compare favourbly with other available detectors in this wavelength range. A simple analystical model was developed to help design and study the corresponding device physics governing the performance of the detectors and was found to give good agreement with the experimentally measured values.