Two-dimensional materials for ultrasonic field-effect transistors

With the increasing miniaturization of electronic components, researchers are struggling with unwanted side effects: In the case of nanometer-scale transistors made of conventional materials such as silicon, quantum effects occur that reduce their functionality. One of these quantum effects is, for example, extra leakage currents, that is, currents that “disappear” and do not flow through the conductor between the source and drive contacts. It is therefore believed that Moore’s scaling law, which states that the number of integrated circuits per unit area doubles every 12-18 months, will reach the limits in the very future due to the increasing challenges associated with the miniaturization of has the active component. This ultimately means that the currently produced silicon-based transistors – called FinFETs and run by almost every supercomputer – can no longer be made arbitrarily smaller due to quantum effects.

Two-dimensional beacons of hope

However, a new study by researchers at ETH Zurich and EPF Lausanne shows that this problem could be overcome with new two-dimensional (2-D) materials – or at least that’s what the simulations they performed on the “Piz Daint” supercomputer suggest.

The research group, led by Mathieu Luisier of the Institute for Integrated Systems (IIS) at ETH Zurich and Nicola Marzari of EPF Lausanne, used the research results that Marzari and his team had already achieved as a basis for their new simulations: Back in 2018, 14 years after the discovery of graphene for the first time made it clear that two-dimensional materials can be produced, they used complex simulations on “Piz Daint” to sift through a pool of more than 100,000 materials; they extracted 1,825 additional components from which 2-D layers of material could be obtained.

The researchers selected 100 candidates from these more than 1,800 materials, each consisting of a monolayer of atoms and could be suitable for the design of ultra-scaled field effect transistors (FETs). They have now examined their properties under the ‘ab initio’ microscope. In other words, they used the CSCS supercomputer “Piz Daint” to first determine the atomic structure of these materials using density functional theory (DFT). They then combined these calculations with a so-called Quantum Transport solver to simulate the electron and hole currents through the virtually generated transistors. The used Quantum Transport Simulator was developed by Luisier together with another ETH research team, and the underlying method was awarded the Gordon Bell Prize in 2019.

Find the optimal 2-D candidate

The deciding factor for the viability of the transistor is whether the current can be optimally controlled by one or more gate contact (s). Due to the ultra-thin nature of 2-D materials – usually darker than a nanometer – a single gate contact can modulate the current of electrons and hole currents, allowing a transistor to turn on and off completely.

Structure of a single port FET with a channel made of a 2-D material. Arranged around are a selection of 2-D materials that have been researched. (Mathieu Luisier / ETH Zurich)

“While all 2-D materials have this property, not all of them lend themselves to logic applications,” emphasizes Luisier, “only those that have a large enough bandgap between the valence band and conduction band.” Material with a suitable bandgap prevents so-called tunnel effects of the electrons and thus the leakage currents caused by them. It is precisely these materials that the researchers searched for in their simulations.

Their goal was to find 2-D materials that can deliver a current greater than 3 milliamperes per micrometer, both as n-type transistors (electron transport) and as p-type transistors (hole transport), and whose channel lengths were similar can be as small as 5 nanometers without limiting the switching behavior. “Only if these conditions are met can transistors based on two-dimensional materials overcome conventional Si FinFETs,” says Luisier.

The ball is now in the court of experimental researchers

In view of these aspects, the researchers identified 13 possible 2-D materials with which future transistors could be built and which could also enable the continuation of Moore’s scaling law. Some of these materials are already known, for example black phosphorus as HfS2, but Luisier insists that others are completely new – compounds such as Ag2N6 or O6Sb4.

“We have created one of the largest databases of transistor materials, thanks to our simulations. With these results we hope to motivate experiments working with 2-D materials to exfoliate new crystals and the next-generation logic circuits. making, ”says the ETH professor. The research groups led by Luisier and Marzari work closely with the National Center for Competence in Research (NCCR) MARVEL and have now published their latest joint results in the journal ACS Nano. They are sure that transistors based on these new materials can replace those made of silicon as well as the currently popular road alkogenides of transition metals.

Provided by National Center for Competence in Research (NCCR) MARVEL