The Hg_(1-x)Cd_xTe photovoltaic detectors with x=0.217 passivated by single ZnS layer ad dual (CdTe+ZnS) layers were fabricated in the same wafer. The fabricated devices were characterized by measurements of the diode low-frequency noise. The diode passivated by dual (CdTe+ZnS) layers show higher performance compared to diode passivated by the single ZnS layer at high reverse bias, and the modeling of diode dark current mechanisms indicate that the performance of the diode passivated by single ZnS is strongly affected by tunneling current related to the surface defects, which is responsible for the low frequency noise characteristics. By the analysis of X-ray reciprocal space map, it was found that the Q_y direction broadening of HgCdTe epitaxial layer passivated by ZnS was wider than the CdTe+ZnS, which confirmed the existence of defects in the surface of HgCdTe epitaxial layer passivated by ZnS.