We study the influence of doping density and the resulting optimum operation voltage on the performance of quantum dot infrared photodetectors (QDIPs). The optimum operation voltage, where detectivity becomes maximum, becomes smaller as the doping density increases. This is because the optimum dark current levels are similar regardless of the doping density. We confirmed experimentally that the optimum dark current level is similar to5 mA (current density: similar toA/cm(2)) for our samples. It is found that the higher doping density improves the performance in the range used in this experiment (5 x 10(16)-5 x 10(17)/cm(3)). The response to a normal incident light is confirmed and the possibility of high-temperature operation of QDIP is shown.