Absolute spectral response modeling of lattice matched Npn InP/In0.53Ga0.47As heterojunction phototransistors (HPTs), for communication wavelength detection, is presented in this paper. Parameters such as collection efficiency, quantum efficiency and doping concentrations affecting the flux absorption profile are discussed. The effect of collection efficiency on the optical responsivity is also highlighted and its variation with device vertical width is discussed. Measured results for optical responsivity, at several incident wavelengths, show close agreement to the modeling data for the HPTs.