Hg_(1-x)Cd_xTe ion-implanted N~+-P gate-controlled photodiodes are fabricated.Measurements show that the P-N junction characteristics are limited seriously by surface channel leakage current due to strong inversion of P-side surface. A theoretical analysis about this leakage mechanism is carried out in detail. By taking into account the unique features of narrow band-gap Hg_(1-x)Cd_xTe, forward and reverse I-V characteristics and the temperature dependence of R_0A product determined by surface channel current are calculated as a function of the gate bias. A qualitative agreement between experimental and the-oretical results is obtained.