Ultimate parameters of HgCdTe and InAsSb n(+)-p photodiodes for the 5-mum spectral region operating at a temperature of 220 K have been determined. Parameters of these devices are limited by noise coming from statistical processes of thermal generation and recombination of carriers. In the case of HgCdTe photodiodes. only two processes are important, Auger 1 and Auger 7, whereas in InAsSb photodiodes the Auger S process is dominant. It is proved that Hg1-xCdxTe and InAs1-xSbx photodiodes constructed optimally have similar detection parameters. In both types of photodiodes, broadening of the energy band gap in the n(+) region leads to a similar, almost twofold increase in the normalized detectivity.