Computational design of tunnel diodes with negative differential resistance and ultrahigh peak-to-valley current ratio based on two-dimensional cold metals: The case of ${\mathrm{Nb}\mathrm{Si}}_2{N}_4/{\mathrm{Hf}\mathrm{Si}}_2{N}_4/{\mathrm{Nb}\mathrm{Si}}_2{N}_4$ lateral heterojunction diode

Cold metals have recently gained attention as a promising platform for innovative devices, such as tunnel diodes with negative differential resistance (NDR) and field-effect transistors with subthreshold swings below the thermionic limit. Recently discovered two-dimensional (2D) ${\text{MA}}_2{Z}_4$ ($M$ = $\mathrm{Ti}$, $\mathrm{Zr}$, $\mathrm{Hf}$, $\mathrm{Nb}$, $\mathrm{Ta}$; $A$ = $\mathrm{Si}$, $\mathrm{Ge}$; $Z$ = $N$, $P$) compounds exhibit both cold-metallic and semiconducting behavior. In this work, we present a computational study of lateral heterojunction tunnel diodes based on 2D ${\mathrm{Nb}\mathrm{Si}}_2{N}_4$ and ${\mathrm{Hf}\mathrm{Si}}_2{N}_4$ compounds. Employing density-functional theory combined with a nonequilibrium Green-function method, we investigate the current-voltage ($I$-$V$) characteristics of lateral tunnel diodes with varying barrier thicknesses in both zigzag and armchair orientations. We find that tunnel diodes in the zigzag orientation exhibit significantly higher peak current densities, while those in the armchair orientation display larger peak-to-valley current ratios (PVCRs) compared to the zigzag orientation. Our findings suggest that ${\text{MA}}_2{Z}_4$ materials are promising candidates for realizing NDR tunnel diodes with ultrahigh PVCR values, which could have potential applications in memory, logic circuits, and other electronic devices.