Theoretical Prediction of Semiconductor-Free Negative Differential Resistance Tunnel Diodes with High Peak-to-Valley Current Ratios Based on Two-Dimensional Cold Metals

The negative differential resistance (NDR) tunnel diodes are promising alternative devices for beyond-CMOS (complementary metal oxide semiconductor) computing because they offer several potential applications when integrated with transistors. We propose a semiconductor-free NDR tunnel diode concept that exhibits an ultrahigh 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 Λ-type NDR effect can be obtained. Two-dimensional (2D) nanomaterials 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 (AlI₂/MgI₂/AlI₂) and vertical (NbS₂/h-BN/NbS₂) heterojunction tunnel diodes based on 2D cold metals. For the lateral tunnel diode, we obtain a Λ-type NDR effect with an ultrahigh PVCR value of 10¹⁶ at room temperature, while the vertical tunnel diode exhibits a conventional N-type NDR effect with a smaller PVCR value of about 10⁴. The proposed concept provides a semiconductor-free solution for NDR devices to achieve the desired I–V characteristics with ultrahigh PVCR values for memory and logic applications.