  The negative differential resistance (NDR) tunnel diodes are promising
alternative devices for beyond-CMOS computing as they offer several potential
applications when integrated with transistors. We propose a novel
semiconductor-free NDR tunnel diode concept that exhibits an ultra-high
peak-to-valley current ratio (PVCR) value. Our proposed NDR diode consists of
two cold metal electrodes separated by a thin insulating tunnel barrier. The
NDR effect stems from the unique electronic band structure of the cold metal
electrodes, i.e., the width of the isolated metallic bands around the Fermi
level as well as the energy gaps separating higher- and lower-lying bands
determine the current-voltage ($I$-$V$) characteristics and the PVCR value of
the tunnel diode. By proper choice of the cold metal electrode materials,
either a conventional N-type or ${\Lambda}$-type NDR effect can be obtained.
Two-dimensional (2D) materials offer a unique platform for the realization of
proposed NDR tunnel diodes. To demonstrate the proof of concept we employ the
nonequilibrium Green function method combined with density functional theory to
calculate the $I$-$V$ characteristic of the lateral (AlI$_2$/MgI$_2$/AlI$_2$)
and vertical (NbS$_2$/h-BN/NbS$_2$) heterojunction tunnel diodes based on 2D
cold metals. For the lateral tunnel diode, we obtain a ${\Lambda}$-type NDR
effect with an ultra-high PVCR value of 10$^{16}$ at room temperature, while
the vertical tunnel diode exhibits a conventional N-type NDR effect with a
smaller PVCR value of about 10$^4$. The proposed concept provides a
semiconductor-free solution for NDR devices to achieve desired $I$-$V$
characteristics with ultra-high PVCR values for memory and logic applications.
