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This paper investigates the dependence of background limited performance (BLIP) temperature of quantum well infrared photodetectors (QWIPs) on doping density. In contrast to the generally accepted optimal doping condition EF=kBTBLIP, our theoretical prediction shows that lower doping densities should slightly increase the BLIP temperature TBLIP taking into account the temperature dependence of the Fermi energy EF, a factor neglected in the previous analyses. Numerical modeling results are used to reinterpret the reported TBLIP measurements for a series of QWIPs of typical design for 9 μm peak wavelength with different doping values. In addition, based on the same general expression for the Fermi energy, the optimized sheet doping concentration to achieve maximum detectivity is given by EF=1.37 kBT, a revision to the previous EF=2 kBT condition.