Topological transistors could enable ultralow-energy electronics

Australian researchers have announced a major advancement towards the creation of a functioning topological transistor, which would burn much less energy than conventional electronics.

Their work, published in the journal Nature, would thus bring us one step closer to minimising the energy wasted by modern telecommunications and computing.

A significant proportion of the growing amount of energy used in information and communications technology (ICT) — such as mobile phones, computers and the data centres that connect them — is caused by transistor ‘switching’. Each time a transistor switches, a tiny amount of energy is burnt — and with trillions of transistors switching billions of times per second, this energy adds up.

For many years, the energy demands of an exponentially growing number of computations were kept in check by ever more efficient and ever more compact CMOS (silicon-based) microchips. But as fundamental physics limits are approached, there are limited future efficiencies to be found.

“For computation to continue to grow, to keep up with changing demands, we need more-efficient electronics,” said study author Professor Michael Fuhrer.

“We need a new type of transistor that burns less energy when it switches.”

Over the last decade, there has been much excitement about the discovery that there are two types of insulators: normal insulators which don’t conduct electricity and topological insulators — newly discovered materials that conduct electricity only on their edges.

Researchers from the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), based at Monash University, have now succeeded in ‘switching’ a material between these two states of matter via application of an electric field. This is the first step in creating a functioning topological transistor.

Topological materials and topological transistors

Topological insulators are novel materials that behave as electrical insulators in their interior, but can carry a current along their edges.

“In these edge paths, electrons can only travel in one direction,” said lead author Dr Mark Edmonds. “And this means there can be no ‘back-scattering’, which is what causes electrical resistance in conventional electrical conductors.”

Unlike conventional electrical conductors, such topological edge paths can carry electrical current with near-zero dissipation of energy, meaning that topological transistors could burn much less energy than conventional electronics. They could also potentially switch must faster.

Topological materials would form a transistor’s active, ‘channel’ component, accomplishing the binary operation used in computing, switching between open (0) and closed (1). The electric field induces a quantum transition from ‘topological’ insulator to conventional insulator.

In order to be a viable alternative to silicon-based technology, topological transistors must operate at room temperature (without the need for expensive supercooling); ‘switch’ between conducting (1) and non-conducting (0); and switch extremely rapidly, by application of an electric field.

“While switchable topological insulators have been proposed in theory, this is the first time that experiment has proved that a material can switch at room temperature, which is crucial for any viable replacement technology,” Dr Edmonds said.

The study

The material Na₃Bi is a topological Dirac semimetal (TDS), which has long been considered a promising system in which to look for topological field-effect switching as it lies at the boundary between conventional and topological phases. The FLEET researchers used scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES) to study the structure and electronic state of Na₃Bi.

Thin film of Na₃Bi grown on sapphire with gold contacts to allow for electrical measurements of the carrier density and mobility at low temperature in a magnetic field.

The study found that when Na₃Bi is made ‘atomically thin’ (ie, only a few layers of atoms in thickness), it is possible to open an electronic band gap, turning the material into an insulator. This bandgap is an essential component in any electronic switch.

Specifically, the researchers found that:

• in the absence of an electrical field, atomically thin Na₃Bi is a 2D topological insulator with a ‘bulk’ (ie, interior) bandgap >300 meV (ie, behaving as an electrical insulator), and no bandgap at the edges (ie, only the edges conduct electricity, and they conduct without dissipation);
• at a critical applied electrical field, the bandgap closes to zero everywhere, and the material becomes a semimetal like graphene (ie, it conducts everywhere — interior and edges);
• above that critical electrical field, the bandgap opens everywhere (to 90 meV) and there is no conduction (interior and edges).
   

“These large bandgaps are much greater than the thermal energy available at room temperature (25 meV, which demonstrates that ultrathin Na3Bi is suitable for room-temperature topological transistor operation,” said study author James Collins.

“Ie, the material is capable of dissipationless transport of charge at room temperature.”

Top image caption: Study authors Dr Mark Edmonds, James Collins and Prof Michael Fuhrer at the School of Physics and Astronomy at Monash University.

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