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Mesa structural n-type nanocrystalline-FeSi2/p-type Si heterojunctions were fabricated by a lift-off technique combined with a photolithography process. The current-voltage characteristics of mesa heterojunctions were measured at low temperatures from 300 K down to 60 K. We investigated their current transport mechanism based on thermionic emission theory. The ideality factor was estimated from the slope of the linear part from the forward lnJ-V characteristics. At 60K, the mesa heterojunctions exhibited the maximum rectification ratio of 106. The ideality factor was calculated to be 1.54 at 300 K and it was increased to be 2.01 at 140 K. The values of ideality factor < 2 indicated that the carrier transport mechanism was governed by a recombination process in the NC-FeSi2 film and at the heterojunction interface. At 120 K, the ideality factor was calculated to be 2.31 and it was increased with decreasing tempearture down to 60 K. The values of ideality factor > 2 indicated that the current transport mechanism was governed by a tunneling process.