Investigation of the performance of GaAs/AlGaAs quantum well IR photoconductors (QWIPs) as compared to HgCdTe photodiodes operated at temperatures below 77 K in the long wavelength IR region are presented. In comparative studies the current standard n -p HgCdTe photodiodes as well as p -n photodiodes are considered. Investigations of fundamental physical limitations of HgCdTe photodiodes indicate better performance of this type of detectors in comparison with QWIPs operated in the range 40 to 77 K. Only at temperature 40 K, QWIPs with a cutoff wavelength of about 8 micrometers indicate higher detectivity. The advantage of QWIPs increases in wider spectral region in lower temperatures - below 40 K. Usually, in the temperature range below 50 K the performance of n -p HgCdTe photodiodes is determined by trap-assisted tunneling. As a result, the advantage of GaAs/AlGaAs QWIPs increases in wider spectral regions ((lambda) approximately equals 8 to 12 micrometers ) and at temperatures below 50 K. GaAs/AlGaAs QWIPs at 40 K are background limited in low-background conditions. This observation plus the maturity of GaAs/AlGaAs technology and its radiation hardness characteristics promise that QWIP technology can produce high- quality focal plane arrays for space applications.