Research Bits: May 16

Germanium-tin transistor

Scientists at Forschungszentrum Jülich, CEA-Leti, University of Leeds, Leibniz Institute for High Performance Microelectronics, and RWTH Aachen University fabricated a new type of transistor from a germanium-tin alloy. Charge carriers can move faster in the material than in silicon or germanium, which enables lower voltages in operation.

“The germanium–tin system we have been testing makes it possible to overcome the physical limitations of silicon technology,” says Qing-Tai Zhao of the Peter Grünberg Institute (PGI-9) at Forschungszentrum Jülich. In experiments, the germanium–tin transistor exhibits an electron mobility that is 2.5 times higher than a comparable transistor made of pure germanium.

The alloy is also compatible with the existing CMOS process for chip fabrication and could be integrated directly into conventional silicon chips.

Electron micrograph of the germanium-tin transistor: The design follows a 3D nanowire geometry that is also used in the latest generation of computer processors. Credit: Forschungszentrum Jülich

The researchers say it has potential for future low-power, high-performance chips. It could also be used in future of quantum computers that integrate parts of the control electronics directly on the quantum chip operating in cryogenic temperatures.

“The challenge is to find a semiconductor whose switching can still be very fast with low voltages at very low temperatures,” said Zhao. For silicon, this switching curve flattens out below 50 Kelvin. Then, the transistors need a high voltage and thus a high power, which ultimately leads to failures of the sensitive quantum bits because of the heating. “Germanium–tin performs better at these temperatures in measurements down to 12 Kelvin, and there are hopes to use the material at even lower temperatures.”

It could also be used in optical on-chip data transmission. Researchers at Jülich have previously developed a germanium-tin laser, which could enable optical data transmission directly on a silicon chip. The germanium-tin transistor, along these lasers, adds the possibility for the monolithic integration of nanoelectronics and photonics on a single chip.

Vertical GeSn nanowire MOSFETs for CMOS beyond silicon: https://doi.org/10.1038/s44172-023-00059-2

Wood transistor

Researchers at Linköping University and the KTH Royal Institute of Technology developed a transistor made of wood. Previous attempts to make a wood transistor were only able to regulate ion transport, and stopped functioning when the ions ran out.

“Yes, the wood transistor is slow and bulky, but it does work, and has huge development potential,” said Isak Engquist, senior associate professor at the Laboratory for Organic Electronics at Linköping University.

The researchers used balsa wood to create their transistor, as the technology involved requires a grainless wood that is evenly structured throughout. They removed the lignin, leaving only long cellulose fibers with channels where the lignin had been. These channels were then filled with a conductive polymer called PEDOT:PSS, resulting in an electrically conductive wood material.

The researchers used this to build the wood transistor and could show that it is able to regulate electric current and provide continuous function at a selected output level. Credit: Thor Balkhed / Linköping University

This was used to build the wood transistor, which was able to regulate electric current and provide continuous function at a selected output level. It could also switch the power on and off, although with a delay of about a second to switch off, and five to switch on.

While there are possible applications in bioelectronics and plant electronics, “We didn’t create the wood transistor with any specific application in mind,” noted Engquist. “We did it because we could. This is basic research, showing that it’s possible, and we hope it will inspire further research that can lead to applications in the future.”

Electrical current modulation in wood electrochemical transistor: https://doi.org/10.1073/pnas.2218380120

Tunable graphene nanocircuits

Scientists from the Center for Research in Biological Chemistry and Molecular Materials (CiQUS), Catalan Institute of Nanoscience and Nanotechnology (ICN2), University of Cantabria, Donostia International Physics Center (DIPC), and Technical University of Denmark (DTU) developed a method for building carbon nanocircuits with tunable properties.

They synthesized a new nanoporous graphene structure by connecting ultra-narrow graphene strips, or “nanoribbons,” by means of flexible “bridges” made of phenylene moieties (which are portions of larger molecules). By modifying in a continuous way the architecture and angle of these bridges, the scientists can control the quantum connectivity between the nanoribbon channels and, ultimately, fine-tune the electronic properties of the graphene nanoarchitecture. The tunability could also be controlled by external stimuli, such as strain or electric fields.

One application could be synthesis of new materials with tailored properties for quantum circuits. It could also be used to develop thermoelectric nanomaterials for renewable energy.

Molecular Bridge Engineering for Tuning Quantum Electronic Transport and Anisotropy in Nanoporous Graphene: https://doi.org/10.1021/jacs.3c00173