Chapter 5 - Selector devices for emerging memories

Abstract

Emerging nonvolatile memory devices, also named memristive devices, have drawn great attention due to their potential for ultra-high density memory application. To achieve such high-density memristive device, a crossbar array structure is introduced. However, implementing such crossbar array requires addressing issues such as unwanted current flow in the unselected cell, so-called the “sneak-path” current. Therefore an access device called a selector device is required to suppress such undesired current and to operate the target memory cell both selectively and nondestructively in the crossbar array. Although different kinds of selector devices have been studied, none of them met all requirements, such as simple two-terminal structure, highly nonlinear I–V characteristics, fast operation speed, high endurance, excellent uniformity, and good thermal stability. The insulator–metal transition (IMT), the ovonic threshold switching (OTS), and the conductive bridging RAM (CBRAM)-type device are promising candidates as a selector device due to their scalability. IMT- and OTS-based selectors exhibit excellent performance, such as high on-current density and extremely fast switching speed, but the off-current, especially for IMT-based selectors, still needs further improvement for the effective reduction of leakage current in the crossbar array. CBRAM-type selectors have ultralow leakage current, suitable for high selectivity and low power device applications. However, the poor on-current density and reliability issues hinder their applicability. There has been great effort to improve the performance of these three types of selector devices, and this chapter reviews recent progress and future directions on their implementation for high-density crossbar array.