We study the electrophysical properties of the Fe/MgO/Fe magnetic tunnel junctions (MTJ) with impurities. Sample structures are fabricated on top of fine-crystalline glass–ceramic substrates by e-beam evaporation in a relatively low vacuum (∼10−4 Torr). The influence of the first magnetic layer fabrication conditions on the degradation of the MTJ is explained by the interlayer diffusion. Various models of electrophysical processes in MTJ on polycrystalline substrates are discussed. The current–voltage (I–V) characteristics of the fabricated structures are found to exhibit a region with negative differential resistance, similar to the one in tunneling diodes. We explain this phenomenon by the formation of excitons in the MgO layer modified by the conductive impurity atoms and their diffusion. The obtained results will be useful in the development of MRAM devices containing MTJs and tunneling diodes.