We study the performance of quantum well infrared photodetectors (QWIPs) in the case of infrared irradiation. This type of photodetector is interesting from the point of view that QWIPs have numerous advantages over photodetectors based on HgCdTe in terms of large array size, high uniformity, high yield, radiation hardness and lower cost of the systems. Therefore, it is important to evaluate their characteristics theoretically. We develop a simple modeling for this interesting type of photodetector. This model describes a nontrivial evaluation of the most important characteristics. The potential distribution of the developed model is obtained by self-consistently solving the Poisson equation. On the other hand, it is used to calculate the dark current, responsivity and detectivity as a function of the structural parameters. These parameters are the spacing between the wells, the number of quantum wells and the operating temperature. Also, the optimization of the characteristics of QWIPs is of primary concern. The effect of uniformity of the dopant density in the QWIP is studied theoretically. We find that the uniformity of the dopant distribution in the plane of QW decreases the dark current.