Abstract: The composition and refractive index of GaInAsSb quaternary semiconductor material for photodetector operating at 2.4μm are obtained by using interpolation and models of dielectric function. By putting the conventional p-i-n structure of GaInAsSb photodetector between two groups of Bragg reflective mirrors, nearly unit quantum efficiency can be reached. Transfer Matrix Method （TMM） is used to calculate the reflectivity of AlAsSb/GaSb Bragg mirrors as a function of wavelength and the number of mirrors. The structure of RCE detector is determined and the influence of the changes of absorption with wavelength on the quantum efficiency is also discussed.