Abstract A new simple carrier density approximation is proposed to take account of carrier degeneracy and conduction band non-parabolicity into the simulation of HgCdTe (Mercury Cadmium Telluride – MCT) photovoltaic devices. It demonstrates a similar accuracy to the numerical calculation of the non-parabolic Fermi–Dirac integration in the range of the reduced Fermi energy at less than 30 for both LWIR and MWIR devices. By using the analytic model of the MCT device simulation, R d– V curve comparisons are made with the carrier density approximations by considering (1) both carrier degeneracy and conduction band non-parabolicity, (2) only carrier degeneracy, (3) only conduction band non-parabolicity, and (4) neither of these two factors. The effects of the omission of carrier degeneracy and conduction band non-parabolicity on each of the four types of dark current mechanisms are evaluated, which is found to lead to an enormous deviation in the simulation of MCT devices, especially for LWIR devices with heavy doping. The deviations of dark current in the latter three cases from the first case under different dominant dark current mechanisms are also demonstrated.