We have first investigated three superlattice infrared photodetectors in order to raise their temperature for the background limited performance. Their basic structure is a 20-period GaAs(6nm)/Al0.32Ga0.78As(4nm) superlattice sandwiched between two 50nm AlxGa1-xAs blocking layers. We changed the barrier height (x) of the blocking layers and the well doping density (Nd) to see their effects on the temperature for the background limited performance. Three samples were grown with molecular beam epitaxy. The associated parameters (x, Nd) of Devices A, B and C are (0.28, 1018), (0.28, 1017) and (0.24, 1017) respectively. The unit for Nd is cm-3. The comparison between A and B is to see the effect of the doping density while that between B and C is the barrier height. The I-V characteristics of the three detectors at various temperatures have been investigated carefully. Two important experimental results have been concluded. A high doping density not only decreases the activation energy and increases the dark current but also increases the impurity scattering for the photoelectrons. For the dark current, high activation energy puts down the dark current and extends the dominant region of the thermionic emission current. Applying those results, we changed the parameters into (0.32, 5 X 1016) and successfully fabricated a superlattice infrared photodetector to reach the background limited performance at 77K. The detectivity for the detector at 77K with wavelength 8.81 micrometers is 4.3 X 109 cm(root)Hz/W.