Reports from the Heart of the Machine
Machine Heart Editorial Department
The size equivalent is different as small as a transistor gate switch.
Since the advent of integrated circuits in the 1950s, silicon transistors have shrunk as predicted by Moore's Law. The number of transistors on microchips is increasing, and the computing power is getting higher and higher.
In recent years, however, transistors have rapidly approached their limits. The length of the gate will soon be unable to shrink any longer, and the "singing" sound of the imminent end of Moore's Law will spread in the chip industry.
In all transistors, current flows from the source to the drain, and this flow of electrons is controlled by a gate that opens and closes based on the applied voltage. Therefore, the length of the gate is a key indicator of the size of the transistor. Below 5 nm, due to the tunneling effect, a quantum physical phenomenon, silicon is no longer able to control the flow of electrons from the source to the drain.
Recently, scientists have begun exploring two-dimensional materials for next-generation electronics, including graphene, which consists of a single layer of carbon atoms, and molybdenum disulfide (MoS2) composed of two layers of sulfur atoms sandwiched between a layer of molybdenum atoms. For example, in 2016, scientists used carbon nanotubes and molybdenum disulfide to make a transistor with a gate length of only 1 nm, but that was not the limit.
Recently, Professor Ren Tianling's team from the School of Integrated Circuits of Tsinghua University has made a major breakthrough in the research of small-size transistors, realizing for the first time a transistor with a sub-1 nanometer gate length and good electrical properties. The study, titled "Vertical MoS2 transistors with sub-1-nm gate lengths," was published in the latest issue of nature magazine.
The corresponding authors of the paper are Professor Ren Tianling and Associate Professor Tian He of the School of Integrated Circuits of Tsinghua University, the co-first authors include Wu Fan, Associate Professor Tian He, and Shenyang, a 2019 doctoral student of the School of Integrated Circuits of Tsinghua University, and other participating authors include Hou Zhan, a 2020 master's student of the School of Integrated Circuits of Tsinghua University, Ren Jie, a 2018 master's student, Gou Guangyang, a 2022 doctoral student, Associate Professor Yang Yi, and Associate Professor Sun Yabin of the School of Communication and Electronic Engineering of East China Normal University.
"We have achieved the smallest gate length transistor in the world," Ren said. The gate length of such transistors is only about one-third of a nanometer wide, which is about the thickness of a single layer of carbon atoms.
Professor Ren Tianling.
To understand this new device, you can imagine two steps of the staircase, the upper step top is the source pole and the top of the lower step is the drain, both of which are made of titanium and palladium metal contacts. The cross-section of the staircase acts as an electronic channel connecting the source and drain, which is made of molybdenum disulfide. Beneath this section is a thin layer of electrically insulated hafnium dioxide.
Figure 1: Schematic diagram of a sub-1 nanometer gate-length transistor structure. Highlights represent the gate electrode of the transistor.
Inside the higher step is a multi-tier sandwich structure. On the ground floor is a sheet of graphene, made up of a single layer of carbon atoms; above it is a block of aluminum covered with alumina that separates graphene and molybdenum disulfide almost completely, except for a thin gap on the vertical side of the higher steps. Both the higher and lower steps are located on the silicon dioxide layer of the 5cm silicon wafer.
The research team cleverly used the ultra-thin single atomic layer thickness and excellent conductivity of the graphene film as the gate, and controlled the switching of the vertical molybdenum disulfide channel through the graphene lateral electric field, so as to achieve an equivalent physical gate length of 0.34 nm, the same width as the graphene layer.
By depositing metallic aluminum on the surface of graphene and naturally oxidizing, the study completed the shielding of the vertical electric field of graphene, using hafnium dioxide deposited in atomic layers as the gate medium and a single-layer two-dimensional molybdenum disulfide film deposited by chemical vapor deposition as a channel. The specific device structure, process flow, and completed physical figure are shown in the following figure:
Figure 2: Sub-1 nanometer gate transistor device process flow, schematic diagram, characterization diagram, and physical diagram.
"In the future, it will be almost impossible to make gate lengths of less than 0.34 nm," professor Ren said. "This may be the last node of Moore's Law."
In 2021, another team of researchers unveiled a vertical transistor they studied, which is made of molybdenum disulfide and has a gate length of 0.65 nm. But Tsinghua's new work pushes the size limit of the gate even further to "the thickness of just one layer of carbon atoms," said Huamin Li, a nanoelectronics scientist at the State University of New York at Buffalo. It was difficult to break this record for quite some time.
In transistors, there is usually a difference in length between the state of gate on and off when an electric field is applied, but on a larger scale, this effect is usually not noticeable. In this new device, when a voltage is applied to the gate to switch it to the off state, the equivalent length of the gate becomes 4.54 nanometers, a difference that can prove to be an advantage.
"In the off state, a long channel with a higher resistance will help prevent leakage current," Li says, "instead, in the on state, a shorter channel with a smaller resistance will increase the current density in the on state."
This work has driven the further development of Moore's Law to the sub-1 nanometer level, while providing a reference for the application of two-dimensional thin films in future integrated circuits.
In the future, Tsinghua researchers plan to create circuits on a larger scale with their new transistors. Ren Tianling said, "The next goal is to make a 1-bit CPU." However, this goal is also challenging, one of which is the manufacture of higher quality, larger areas of molybdenum disulfide, and the currently high cost of the material.
All in all, "this prototype work is a new attempt by humans to explore transistor vertical architectures following the development of FinFET technology," Li said, "and hopes that this will inspire more creative ideas to fully explore the potential of 2D materials and extend Moore's Law to the field of high-performance, energy-efficient nanoelectronics."
Reference Links:
https://spectrum.ieee.org/smallest-transistor-one-carbon-atom
https://www.tsinghua.edu.cn/info/1175/92075.htm
Beijing, March 23 – Chief Chi Heng Executive Conference
The Heart of Machines AI Technology Annual Conference will be held on March 23, along with the Chief Chi Heng Conference.
Time: March 23, 2022, 13:30-17:00
Address: Hyatt Regency Beijing Wangjing
The "Chief Intellectual Officer Conference" will invite leaders in the field of smart travel, who will come from the most popular smart cars, vehicle specification chips, Robotaxi and unmanned logistics and other fields, covering a number of cutting-edge directions such as automotive robots, automotive chip prospects in the era of large computing power, and driverless commercialization.