  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) MA$_2$Z$_4$ (M = Ti,
Zr, Hf, Nb, Ta; A = Si, 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 NbSi$_2$N$_4$ and
HfSi$_2$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 MA$_2$Z$_4$ materials are promising candidates for
realizing NDR tunnel diodes with high PVCR values, which could have potential
applications in memory, logic circuits, and other electronic devices.
