The dark current characteristics of long-wavelength HgCdTe were analyzed, compared upon three types of devices. By I-V measurement under different temperatures and dark current mechanisms, the dominant currents of each device were clarified at different temperatures. It is demonstrated that the B+-implanted n+-on-p planar junction on silicon substrate is comparable to that on bulk cadmium zinc telluride (CdZnTe) substrate above 80K. However, the trap-assisted tunneling current becomes dominant under 80K due to the high density of dislocations. Compared with n+-on-p junctions, the p+-on-n double-layer heterojunction inhibits the diffusion current effectively, which is good matched with the calculation result upon the parameter, derived from I-V curve fitting. This p+-on-n diode has a R0A value of 38 Ω·cm2 at 80K, for the cut-off wavelength of 9.6 μm, while that of the n+-on-p diode on bulk CdZnTe is 2.5 Ω·cm2. Below 60K, the dislocations make the R0A value of the p+-on-n diode an order of magnitude lower than that of the n+-on-p diode on CdZnTe.