For decades the transistors in microchips became smaller and smaller. About every two years the number of transistors on commercially available chips could be doubled – this phenomenon became known as “Moore’s Law”. But for some years now this has been a thing of the past. Miniaturization has reached a natural limit because, on a scale of a few nanometers, one suddenly has to contend with completely new problems.
Now, however, the next major miniaturization step could become possible – with two-dimensional (2D) materials consisting only of an atom-thick material layer. With the help of a novel insulator made of calcium fluoride, the University of Technology (TU) of Vienna has now succeeded in producing an ultra-thin transistor that has excellent electrical properties and, in contrast to previous technologies, can also be extremely reduced in size due to its low thickness.
Research on semiconductor materials, as required for the manufacture of transistors, has made great progress in recent years: In the meantime, ultra-thin semiconductors can be produced as so-called 2D materials consisting of only a few atomic layers. However, this is not enough to build an extremely small transistor. “In addition to the ultra-thin semiconductor, an ultra-thin insulator is also needed, explains Prof. Tibor Grasser from the Institute of Microelectronics at Vienna University of Technology.
This is due to the basic structure of a transistor: current can flow from one side of the transistor to the other – but only if a suitable electric field is generated in the middle by applying an electric voltage. An insulating layer is needed between the electrode that provides this field and the semiconductor itself. “There have always been transistor experiments with ultra-thin semiconductors coupled with ordinary, thicker insulators,” explains Grasser. “But that is of little use – first, we cannot speak of miniaturization if the transistor, including the insulator, has a greater thickness after all, and second, it became apparent that the sensitive electronic properties of the semiconductor are disturbed by the inferior insulator surface.
Yury Illarionov, postdoc in Grasser’s team, therefore pursued a novel approach: “If an ultra-thin material with a clearly defined surface is used not only for the semiconductor but also for the insulator, for example ionic crystals, then a transistor with a size of only a few nanometers can be built. The electronic properties are improved because ionic crystals have a perfectly regular surface – without individual protruding atoms that could disturb the electric field. “Conventional materials have covalent bonds in the third dimension, i.e. atoms that couple at the top and bottom to atoms of the neighbouring material,” explains Grasser. “This is not the case with 2D materials and ionic crystals, so they do not interfere with the electrical properties of the neighboring material.
An insulator consisting of an atomically thin layer of calcium fluoride was chosen. This layer was produced at the Joffe Institute in St. Petersburg, where the first author of the publication, Yury Illarionov, had also done research, before moving to the TU Vienna. The transistor itself was then manufactured at the Institute of Photonics of the TU Vienna in the group of Prof. Thomas Müller and subsequently examined at the Institute of Microelectronics.
Even the first prototype exceeded all expectations: “In recent years, we have repeatedly received different transistors to investigate their technical properties – but we have never seen anything like our transistor with calcium fluoride insulator,” says Grasser. “With its electrical properties, the prototype outshines all previous results.
Now the scientists want to investigate which combinations of insulators and semiconductors work best. It will probably be some years before the technology can be used for commercially available computer chips, because the manufacturing processes for the material layers still have to be improved. “However, there is no doubt in our minds 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 presented here are currently the best solution to the insulator problem.
For the computer industry, such smaller, faster transistors should make the next major development step possible.