Quantum Tunnelling Transistor Shatters the 60-Millivolt Barrier Holding Back Computer Chips

For more than half a century, the relentless march of computing has been powered by a simple trick: making transistors smaller, faster, and more energy-efficient with each successive generation of integrated circuits. But that march has quietly been running into a wall. A fundamental physical constraint known as the Boltzmann limit, sometimes colourfully described by engineers as Boltzmann tyranny, caps how efficiently a conventional transistor can switch between its ON and OFF states. Now a research team at The Hong Kong Polytechnic University (PolyU), working with collaborators across Singapore and mainland China, has demonstrated a quantum-tunnelling field-effect transistor built from two-dimensional nanomaterials that pushes decisively past that wall, achieving switching behaviour that conventional semiconductor physics has long declared impossible at room temperature.

The breakthrough, published in the prestigious international journal Science, was led by Prof. Jianhua Hao, Head of the Department of Physics and Materials, Chair Professor of Materials Physics and Devices and Associate Director of the PolyU-Wuhan Technology and Innovation Research Institute at PolyU. The collaboration included researchers from the National University of Singapore, The Hong Kong University of Science and Technology, Peking University, and the Singapore University of Technology and Design. Dr. Zehan Wu, Research Assistant Professor in the same department, is the first author of the research article. Their work addresses what the International Roadmap for Devices and Systems (IRDS) has identified as one of the most pressing bottlenecks in modern microelectronics, and it does so using a device architecture that the roadmap itself has flagged as the most promising successor to today’s dominant transistor design.

To appreciate why this matters, it helps to understand how the transistors inside every laptop, smartphone, and data centre actually work. Integrated circuits are built from complementary metal–oxide–semiconductor field-effect transistors, or MOSFETs, which switch electrical current by raising or lowering an energy barrier. In the OFF state, charges face a barrier they cannot cross; when a gating voltage is applied, charges gain enough thermal energy to spill over the top of that barrier in a process called thermionic emission. The steepness of this transition, measured by a figure known as the subthreshold swing (SS), determines how much voltage is needed to flip the transistor from OFF to ON. Here nature intervenes: because thermionic emission depends on the thermal distribution of electron energies, the subthreshold swing of any MOSFET cannot fall below 60 millivolts per decade of current at room temperature. That is the Boltzmann limit, and it is not an engineering shortfall but a consequence of statistical mechanics itself.

The consequences of this limit are profound. As chipmakers pack more transistors onto each processor, the 60 mV decade⁻¹ floor forces operating voltages to remain far higher than ideal, which means wasted power, wasted heat, and a hard ceiling on energy efficiency. Advanced MOSFETs typically require gate-voltage ranges of around 800 millivolts to operate, and every millivolt of that overhead is multiplied billions of times across a modern chip. For the artificial intelligence accelerators now driving enormous growth in global electricity consumption, the inability to switch transistors at lower voltages has become one of the defining engineering challenges of the decade. Breaking the 60 mV barrier is therefore not an incremental improvement but a potential inflection point for the entire industry.

The escape route identified by physicists is quantum tunnelling. Instead of forcing electrons to climb over an energy barrier, a tunnelling field-effect transistor (TFET) allows them to pass straight through it, exploiting the quantum-mechanical probability that particles can traverse barriers that would classically be impenetrable. Because tunnelling probability depends on the barrier’s width and height rather than on the thermal energy of the electrons, a TFET can in principle achieve subthreshold swings steeper than 60 mV decade⁻¹ even at room temperature. The IRDS has singled out TFETs as the most promising alternative to MOSFETs precisely for this reason. In practice, however, experimental TFETs have struggled for years with a frustrating trade-off: devices that achieved steep switching typically delivered painfully low output currents, undermining their usefulness in real circuits.

Prof. Hao’s team solved this problem with an elegantly engineered material system. Using pulsed laser deposition (PLD), a technique in which intense laser pulses vaporise a target material so that it recondenses as an ultra-thin film, the researchers fabricated an alternating heterostructure of two-dimensional bismuth (Bi) and indium selenide (InSe) layers. The choice of bismuth is the crucial insight. In its bulk form, bismuth is a semi-metal, a material that is neither a proper conductor nor a proper semiconductor. But when reduced to two-dimensional form with precise control over the layer structure at the nanoscale, bismuth transforms into a semiconductor. This transformation allowed the team to engineer an ideal energy band alignment between the bismuth and the indium selenide, creating exactly the conditions needed for charge carriers to tunnel efficiently from one material into the other through the quantum tunnelling mechanism.

The performance figures reported in the study are striking. The resulting Bi/InSe TFET achieved subthreshold swing values well below the 60 mV decade⁻¹ thermionic limit, and it maintained this steep switching behaviour across six orders of magnitude of current, a range that matters enormously for practical digital logic, where transistors must remain reliably OFF across many conditions rather than only at a single operating point. Remarkably, the device accomplished all of this at room temperature, on standard centimetre-scale silicon substrates, without exotic cryogenic cooling or specialised wafer treatments. The operating gate-voltage range required was only 160 millivolts, a fraction of the roughly 800 millivolts demanded by advanced MOSFETs, translating directly into the kind of power savings that next-generation computing demands.

Just as importantly, the device resolved the long-standing weakness of previous experimental TFETs. The PolyU-led team demonstrated a high output current of up to several microamps per micrometre alongside an exceptionally high ON/OFF current ratio. Output current is not a vanity metric: it determines whether a single transistor can drive multiple downstream logic gates, a capability engineers call fan-out, and whether signals propagate through a circuit with minimal delay. A steep-switching transistor that cannot deliver adequate current would force designers into cumbersome workarounds and erode the very efficiency gains it promises. By combining steep slopes with robust current drive, the Bi/InSe device demonstrates compatibility with existing integrated-circuit design practices and even the potential for a generational upgrade of current chips rather than a disruptive replacement.

The manufacturing dimension of the work may prove as consequential as the physics. Pulsed laser deposition has long been valued in research laboratories for its precision, but questions have lingered about whether it can serve high-precision, wafer-scale production of two-dimensional materials. This study demonstrates the practical viability of PLD for exactly that role, producing uniform ultra-thin heterostructures on silicon substrates that are the backbone of the existing semiconductor industry. Because the device integrates seamlessly with traditional silicon-based manufacturing processes, the researchers argue that their approach provides a scalable roadmap toward energy-efficient microchips, rather than a laboratory curiosity that would require an entirely new industrial ecosystem to deploy. That compatibility with ultra-short channel lengths, which two-dimensional materials are particularly well suited to support, positions the technology for the transistor geometries that future process nodes will demand.

The implications extend well beyond the laboratory bench. Ultra-low-power, high-performance integrated circuits built on tunnelling transistors could reshape the energy economics of artificial intelligence, where specialised hardware already consumes power on the scale of entire cities, and where every reduction in switching voltage compounds across billions of operations per second. Prof. Hao has emphasised that the technology paves the way for the ICs essential to emerging AI chips and advanced semiconductor applications, and the publication of the findings in Science signals that the broader research community regards the result as a landmark. For decades, the Boltzmann limit has stood as an immovable fact of life for chip designers, a boundary drawn by thermodynamics itself. By replacing thermionic emission with quantum tunnelling in a carefully crafted two-dimensional heterostructure, the PolyU-led team has shown that the boundary can be crossed, and in doing so has brought a long-awaited experimental technology a decisive step closer to commercial reality.

Subject of Research: A quantum-tunnelling field-effect transistor based on two-dimensional bismuth and indium selenide heterostructures that overcomes the Boltzmann switching limit in integrated circuits

Article Title: PolyU develops quantum-tunnelling field-effect transistor to overcome barriers to integrated-circuit chip development

Article References: PolyU develops quantum-tunnelling field-effect transistor to overcome barriers to integrated-circuit chip development. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum tunnelling, tunnelling field-effect transistor, TFET, two-dimensional materials, bismuth, indium selenide, Boltzmann limit, subthreshold swing, pulsed laser deposition, integrated circuits, low-power electronics, AI chips