The National Science and Technology Council (NSTC) of Taiwan has long been committed to advancing critical semiconductor technologies for the post-Moore era through its "Ångström-Era Advanced Semiconductor Technology Program." With NSTC support, Professor Ya-Ping Chiu of the Department of Physics at Na-tional Taiwan University (NTU) pioneered the Operando Cross-Sectional Scanning Tunneling Microscopy (Operando XSTM) technique. In collaboration with Profes-sor Yann-Wen Lan of the Department of Physics at National Taiwan Normal Uni-versity (NTNU) and Professor Lain-Jong Li of the National University of Singapore (NUS), the team has, for the first time, directly measured the carrier transfer length at the metal/semiconductor contact edge of advanced two-dimensional (2D) semiconductor transistors under actual device operating conditions, with atomic-scale spatial resolution. This breakthrough overcomes the long-standing limitation of relying solely on theoretical simulations and indirect extrapolations, providing the first direct experimental evidence for evaluating transistor scaling potential. The findings were published on July 1, 2026, in Nature, one of the world's leading scientific journals.
As demand for high-performance, low-power chips continues to surge, driven by artificial intelligence, data centers, and mobile devices, the miniaturization of semiconductor devices has become a central challenge in global technology de-velopment. Two-dimensional semiconductors, with their atomic-scale thickness and excellent gate electrostatics, are considered leading candidates for extending advanced logic device scaling into the post-Moore era. However, the continued miniaturization and practical application of transistors depends not only on chan-nel length scaling, but equally on the quality of the metal contact region.
The contact edge is the critical zone where electrons are injected from the metal electrode into the semiconductor channel. The carrier transfer length, defined as the effective spatial extent over which this injection occurs, directly governs con-tact resistance and carrier injection efficiency, and is a key indicator of whether a device can sustain normal operation under extreme scaling conditions. Conven-tional theoretical models rely on assumptions that are fundamentally unsuitable for advanced 2D semiconductor transistor systems, and cannot directly capture the true spatial electron transport behavior at the contact interface. As a result, this critical physical parameter has long lacked direct sub-nanometer experimental verification.
This breakthrough builds on nearly two decades of in-house expertise in cross-sectional scanning tunneling microscopy cultivated in Taiwan. The team further integrated an operando biasing capability into the characterization system, ena-bling real-time, atomic-resolution probing of electron transport behavior at the metal/semiconductor contact edge while the device is under actual bias opera-tion. This technique is akin to installing an atomic-scale, high-resolution camera at the contact edge of a transistor, allowing researchers, for the first time, to directly observe how electrons traverse the contact boundary during device operation and to precisely measure the effective carrier transfer length. What was previously ac-cessible only through theoretical estimation can now be directly verified by exper-iment, providing concrete quantitative data for next-generation semiconductor scaling research.
Using a bismuth (Bi) metal contact on a monolayer molybdenum disulfide (MoS₂) transistor as the test platform, the team cleaved the device under ultra-high vacuum conditions, exposing a clean cross-section that included the metal contact, the 2D semiconductor channel, and the dielectric substrate directly in front of the measurement probe. With the device under operational bias, the team directly mapped the local spatial distribution of electrons near the contact edge.
This work carries several important implications. It provides the first atomic-resolution experimental confirmation that 2D semiconductor contact engineering has the genuine potential to support next-generation nanoscale technology nodes. It delivers direct experimental benchmarks for comparing different contact metal and semiconductor material combinations, replacing material screening ap-proaches that have been heavily reliant on simulation. It also equips the semicon-ductor industry with more accurate methods for characterizing contact quality in devices, helping to shorten the validation cycle from research and development to process integration.
Beyond validating the key physical mechanisms at 2D semiconductor contact in-terfaces, the team also successfully extended this measurement approach to sili-con-on-insulator (SOI) devices, demonstrating that Operando XSTM is not limited to 2D materials but holds promise as a universal analytical platform for studying contact properties across diverse advanced semiconductor systems. As transistor dimensions continue to shrink, the ability to precisely characterize electron transport at the contact interface will become increasingly critical to overcoming device performance bottlenecks. The direct measurement capability established by this technique is poised to become a potentially invaluable characterization tool in the development of next-generation semiconductor devices.
This research showcases Taiwan's integrated strength in advanced semiconduc-tor metrology, 2D material device fabrication, and interface physics. It exemplifies an international collaboration model led by Taiwanese researchers, spanning ma-terials science, device engineering, precision measurement, and theoretical analy-sis. From the development of the characterization technique and establishment of the measurement methodology to validation on state-of-the-art devices, the work demonstrates Taiwan's autonomous research and development capabilities and international leadership in frontier semiconductor metrology.
Media Contact：
Dr. Ting-Yang Kuo
Program Manager/Assistant Research Fellow
Department of Natural Sciences and Sustainable Development
National Science and Technology Council
Tel: +886 (2)2737-7465
E-mail: tykuo@nstc.gov.tw
