This is a schematic of the new ultra-thin transistor: the insulator in red and blue, and the semiconductor above. Image: TU Wien.
This is a schematic of the new ultra-thin transistor: the insulator in red and blue, and the semiconductor above. Image: TU Wien.

For decades, the transistors on our microchips have become smaller, faster and cheaper. Approximately every two years, the number of transistors on commercial chips has doubled – this phenomenon became known as ‘Moore's law’. But for the past few years, Moore's law has stopped holding sway. Miniaturization has reached a natural limit, as completely new problems arise when a length scale of only a few nanometers is approached.

Now, however, the next big miniaturization step could soon become possible – thanks to so-called ‘two-dimensional (2D) materials’ that consist of a single atomic or molecular layer. With the help of a novel insulator made of calcium fluoride, scientists at the Vienna University of Technology (TU Wien) in Austria have created an ultra-thin transistor with excellent electrical properties that, in contrast to previous technologies, can be miniaturized to an extremely small size. They report this work in a paper in Nature Electronics.

Research on novel semiconductor materials for fabricating transistors has seen significant progress in recent years. Today, ultra-thin semiconductors can be made from 2D materials consisting of only a few atomic layers. "But this is not enough to build an extremely small transistor," says Tibor Grasser from the Institute of Microelectronics at TU Wien. "In addition to the ultra-thin semiconductor, we also need an ultra-thin insulator."

This is due to the fundamental design of a transistor: current can flow from one side of the transistor to the other, but only if a voltage is applied in the middle, creating an electric field. The electrode providing this field must be electrically insulated from the semiconductor itself.

"There have already been transistor experiments with ultra-thin semiconductors, but until now they were coupled with ordinary insulators," says Grasser. "There is not much benefit in reducing the thickness of the semiconductor when it still has to be combined with a thick layer of insulator material. There is no way of miniaturizing such a transistor any further. Also, at very small length scales, the insulator surface turned out to disturb the electronic properties of the semiconductor."

Therefore, Yury Illarionov, a postdoc in Grasser's team, tried a novel approach. He used ultra-thin 2D materials not only for the semiconductor part of the transistor, but also for the insulating part. By selecting ultra-thin insulating materials such as ionic crystals, a transistor with a size of only a few nanometers can be built. The electronic properties are improved because ionic crystals can have a perfectly regular surface, without a single atom protruding from the surface to disturb the electric field.

"Conventional materials have covalent bonds in the third dimension – atoms that couple to the neighboring materials above and below," explains Grasser. "This is not the case in 2D materials and ionic crystals, and so they do not interfere with the electrical properties of the semiconductor."

To produce the new ultra-thin transistor, the scientists selected calcium fluoride as the insulating material. The calcium fluoride layer was produced at the Ioffe Institute in St. Petersburg, Russia, where the first author of the publication, Yury Illarionov, originally came from before joining the team at TU Wien. The transistor itself was then manufactured by Thomas Müller's team at the Institute of Photonics at TU Wien and analyzed at the Institute for Microelectronics.

The very first prototype has already surpassed all expectations. "For years, we have received quite a number of different transistors to investigate their technical properties – but we have never seen anything like our transistor with the calcium fluoride insulator," says Grasser. "The prototype with its superior electrical properties outshines all previous models."

Now, the team wants to find out which combinations of insulators and semiconductors work best. It may take a few more years before the technology can be used for commercial computer chips, as the manufacturing processes for the material layers still need to be improved.

"In general, however, there is no doubt that transistors made of 2D materials are a highly interesting option for the future," says Grasser. "From a scientific point of view, it is clear that the fluorides we have just tested are currently the best solution for the insulator problem. Now, only a few technical questions remain to be answered. "

This new kind of smaller and faster transistor should allow the computer industry to take the next big step. This way, Moore's law of exponentially increasing computer power could soon come to life again.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.