In this paper, we experimentally charactize -type nanocrystalline iron disilicide films (NC-), grown on -type single-crystalline Si substrates at room temperature by a facing-target direct-current sputtering method. These heterojunction devices are promising candidates for near-infrared photodiode applications. However, their current-voltage characteristics show large leakage currents, which limit the device functions. Hence, we have calculated the energy band diagram of the NC- heterojunction, for the first time, to gain insight into the source of this leakage. The conduction and valance band discontinuities are calculated as 0.19 and 0.46 eV, respectively. A built-in potential of 1 V is estimated from the energy band diagram. This estimate is almost twice the value obtained from the measured capacitance-voltage characteristics. This indicates the presence of a large number of defects at the heterojunction interface. From a simulation combined with the experimental results, the defect density is estimated to be at least . The simulation model successfully reproduces the experimental current-voltage characteristics of the device and reveals that acceptor-like interface states centered at approximately 0.35 eV above the valance band are the main sources of the heterojunction leakage. Simulation results predict that passivation of these defects would reduce the leakage current to 10 , causing - ignificant improvements in the device's performance.