We present the results of a model for the detectivity simulation of multi-period AlGaAs/GaAs Long-wavelength quantum well infrared photodetectors (LW-QWIPs) by assuming a three-dimensional carrier transport in the barriers where the electrical fields are obtained in a self-consistent way. This model takes into account the fundamental mechanisms involved in the device detection process. The electrical field distribution, currents and detectivities are carefully calculated and analyzed. The numerical results explain well our experimental observations.