Two-Dimensional Tunnel Junctions Emerge as Universal Selectors for Memory Arrays

The relentless demand for data storage and processing has pushed memory technology into an era where density, speed and energy efficiency must improve simultaneously. One of the most promising routes toward ultra-dense memory is the crossbar array, in which memory elements sit at the intersections of two perpendicular sets of electrodes, forming a lattice that can, in principle, pack storage cells at the smallest possible feature size. Yet the crossbar architecture carries a stubborn electrical problem: in a passive array without any additional switching element, current does not respect the boundaries of the intended cell. It leaks through neighbouring devices, corrupting read operations and overwhelming the circuitry that must distinguish a stored bit from parasitic background current.

The standard remedy is to pair each memory element with a selector, a two-terminal device that conducts strongly only when a sufficient voltage is applied across it and remains nearly insulating otherwise. The quality of a selector is judged by its nonlinearity, the ratio between the current it passes in the on-state and the current it leaks in the off-state at the same operating conditions. High nonlinearity suppresses sneak paths, but achieving it without sacrificing drive current, switching speed or endurance has proved remarkably difficult. Threshold switches, ovonic devices, mixed-ionic-electronic-conduction devices and transistor-based access elements each bring trade-offs in variability, fabrication complexity or compatibility with the memory stack above them.

Against this backdrop, a News and Views commentary by Yuting Sun and Min Zhu of Shanghai Jiao Tong University, published in Nature Electronics, highlights recent progress that may reshape the selector landscape. The commentary discusses work demonstrating tunnel junction selectors built from five vertically stacked layers of two-dimensional materials, a device architecture that delivers high nonlinearity, robust endurance, fast switching speeds and low device-to-device variability. The authors suggest that such devices could potentially function as a common access element across multiple memory technologies, a prospect that would simplify the design of hybrid memory systems considerably.

Two-dimensional materials, the family of crystals that can be isolated in layers only a few atoms thick, have long been touted for electronic applications because their surfaces are free of the dangling bonds that plague conventional semiconductors at scaled dimensions. In a vertical stack, dissimilar two-dimensional layers can be assembled atom by atom, allowing engineers to engineer the band alignment across the junction with a precision that bulk heterostructures cannot easily match. A tunnel junction exploits this control directly: the barrier formed between the layers governs quantum-mechanical tunnelling, so a modest change in the voltage across the stack can swing the current through many orders of magnitude. That voltage-dependent tunnelling is precisely the behaviour a selector needs.

The five-layer architecture is central to the performance reported. By sandwiching tunnelling barriers between conducting and semiconducting two-dimensional sheets, the device creates an energy landscape in which current transport is strongly suppressed at low bias but rises steeply once the applied voltage exceeds a threshold. The steepness of this transition determines the nonlinearity, and the atomically thin layers mean the effective barrier width is set with atomic precision rather than by the statistical roughness that afflicts amorphous oxide films. Because the layers are crystalline and their interfaces are abrupt, device-to-device variation, historically the Achilles heel of selector technology, is kept low. Variability matters enormously in practice: a crossbar array may contain millions or billions of cells, and the array can only be read reliably if the statistical spread of selector behaviour remains narrow across the entire population.

Endurance and speed are the other pillars of the achievement. A selector in a working memory must withstand billions of switching cycles without degradation, since it is cycled every time the adjacent memory cell is written or read. It must also switch quickly enough not to become the bottleneck of the memory operation. The commentary notes that the two-dimensional tunnel junction selectors exhibit robust endurance and fast switching speeds, indicating that the physical mechanism underlying the switching, governed by electronic tunnelling rather than by the movement of ions or the formation and dissolution of conductive filaments, is inherently less prone to the wear mechanisms that limit other selector types. Filamentary devices, for example, rely on stochastic ionic processes that both slow the switch and gradually rearrange the material, whereas a tunnelling barrier responds on the timescale of electronic transport itself.

The significance of the work extends beyond any single memory technology. Resistive random-access memory, phase-change memory, conductive-bridge memory and ferroelectric memory all require selectors, but each has historically demanded a selector tailored to its particular voltage window, current range and integration scheme. A universal selector, one device that can serve as the access element across several memory platforms, would allow foundries to standardise a single back-end-of-line module and would give memory architects freedom to mix and match storage elements within a system without redesigning the array infrastructure. Sun and Zhu point to this cross-technology compatibility as the potentially transformative aspect of the two-dimensional tunnel junction approach, framing it as a common access element for multiple memory technologies.

The commentary situates the new devices within a broader arc of two-dimensional materials research. Since the isolation of graphene opened the field, researchers have assembled van der Waals heterostructures in which layers are stacked without the lattice-matching constraints of epitaxial growth. Reviews of the field have emphasised that this assembly freedom, combined with the diversity of available two-dimensional crystals, from semiconducting transition-metal dichalcogenides to insulating hexagonal boron nitride, enables device concepts that are difficult or impossible in conventional material systems. The tunnel junction selector is a compelling example of that promise translated into a device metric, nonlinearity, that the memory industry can measure and value directly.

Challenges remain before such selectors can leave the laboratory. Integrating two-dimensional layers into large-area manufacturing remains an unsolved problem at scale, since the growth and transfer of atomically thin films over full wafers must achieve uniformity comparable to that of deposited oxides. Contact resistance between the two-dimensional stack and conventional metal interconnects, thermal stability during back-end processing, and the yield of defect-free multilayer stacks are all active areas of investigation. The commentary’s authors, who are themselves affiliated with a school of integrated circuits and a national key laboratory of micro and nano manufacture technology, are well placed to appreciate the distance between a demonstrated device and a manufacturable module.

Even so, the demonstration marks a notable convergence of two research communities. Memory engineers have spent two decades searching for a selector that combines the nonlinearity of a diode with the bidirectional operation and simple two-terminal structure that crossbar arrays require, while two-dimensional materials researchers have sought applications in which atomically thin layers provide a decisive advantage rather than an incremental one. The tunnel junction selector, with its high nonlinearity, robust endurance, fast switching and low variability, offers evidence that this convergence can produce devices that matter to the industry. If the approach survives the rigours of large-area integration, the humble selector, long an afterthought in memory design, may become the element that finally unlocks the full density promise of three-dimensional crossbar memories.

Subject of Research: Two-dimensional-material tunnel junction selectors for crossbar memory arrays

Article Title: Universal selectors from two-dimensional materials

Article References: Sun, Y., & Zhu, M. (2026). Universal selectors from two-dimensional materials. Nature Electronics. https://doi.org/10.1038/s41928-026-01723-z

Image Credits: AI Generated

DOI: 10.1038/s41928-026-01723-z

Keywords: two-dimensional materials, tunnel junction, selector device, crossbar arrays, memory technology, nonlinearity, endurance, switching speed, device variability, van der Waals heterostructures, Nature Electronics, resistive memory