It has been found from our previous studies that there is no Landau damping in N-type doped narrow-gap HgCdTe, which is contrary to the existing theory and experimental fact. The further study on the effect of doping and hence the degeneration on the dispersion relation of energy of free-carriers and the calculation of Lindhard function of free-carriers taking into account this effect show that the absence of Landau damping results from the change of free-carrier behavior caused by the dispersion relation of energy of free-carriers.