A 3~5㎛ $Hg_{0.7}Cd_{0.3}$ Te infrared photodiode han been fabricated. $n^+$-p junction is formed by boron ion implantation with an energy of 100keV and dose of $1.0\times10^{14}cm^{-2}$. The series resistance of the diode is about 60$\Omega$ and its $R_0A$ product is $1.8\Omega·cm^2$. The diode shows high reverse leakage current which is larger at liquid nitrogen temperature than at room temperature. Its dominant cause is interband tunneling mechanism. Modeling and calculation of the reverse leakage current indicates that substrate concentration of the order of $10^{17}cm^{-3}$ is inappropriate for diode fabrication due to the high tunneling current. Substrate concentration lower than $10^{16}cm^{-3}$ is suggested for the fabrication of high performance devices.
