The effect of a carbon dopant introduction into iron nanofilm on the electrical properties of a metal–semiconductor system is considered. The objects of study are the samples on a $p$- and $n$-types semiconductor substrates of 16 contacts with a diameter of 2 mm and a thickness of up to 100 nm, eight of which are from pure iron and eight—from iron doped with carbon. As shown, for doped iron on the $n$-type substrate, a rectifying contact such as a Schottky diode occurs, which is associated with an increase in the work function of the metal. The height of a potential barrier arising due to this increase in the work function is calculated. The results obtained in the calculation explain the appearance of a region with negative differential resistance on the current–voltage characteristics of the Fe$\backslash$MgO$\backslash$Fe system when one layer of a ferromagnet is doped with carbon.