The electrical excitation of surface plasmons/photons from MIM tunnel junctions has attracted significant research attention in recent times [1]-[3]. These devices, though are promising for future on-chip plasmonic applications, are plagued with low quantum efficiencies. In an effort towards increasing the effectiveness of such devices, we study the behaviour of gap plasmons in Ag-SiO2-Ag and Al-Al2O3-Au structures, with a 3 nm oxide and confined in two dimensions. We find that the out-coupling of leaky modes due to the Fabry-Pérot resonances in the oxide region contributes to the lower efficiencies of MIM tunnel junctions. © 2019 IEEE.