Dynamic storage time has been measured for x=0.31 n-type Hg1-xCdxTe metal-insulator semiconductor (MIS) devices over temperature range of 68 to 250K. The theoretical calculations have been made in analysing the mechanisms of dark currents. The results point out that the indirect tunnel current, which involves a thermal transition of an electron from the valence band to a band-gap state followed by tunneling into the conduction band, is the main dark current source, especially in low-temperature (about 77K) high-field regime and will dominate the electrical properties of the MIS devices.