In 1965 R&D Director at Fairchild (and later Intel co-founder) Gordon Moore predicted continued systemic declines in cost and increase in performance of integrated circuits in his paper “Cramming more components onto integrated circuits.”Moore’s Law which stated that the number of transistors on a chip will double approximately every two years has been the driver of semiconductor industry in boosting the complexity, computational performance and energy efficiency while reducing cost. It has led to substantial improvements in economic productivity and overall quality of life through proliferation of computers, communication, and other industrial and consumer electronics. Microelectronics and solid state components have also been the backbone of the military systems and were main contributors in advancement of radar, communication and electronic warfare systems.
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 was kept in check by ever-more efficient, and ever-more compact CMOS (silicon based) microchips — an effect related to the famous ‘Moore’s Law’.
However as dimensions approach nanometer ranges, CMOS transistors are difficult to operate because of rising power dissipation of chips and the fall in power gain of smaller transistors, soaring fabrication plant costs and finally, quantum effects in silicon will bring about an end to the ongoing Moore’s Law. Today, the standard length of transistors are 10 nanometers, and with the latest research, top companies have produced 5 nm or 7 nm chips. Intel’s chips in production measure 10 nm while TSMC is producing 5nm already.
Transistors smaller than 7 nm experience quantum tunnelling through their logic gates. The switching energy is approaching the thermal noise spectral density. In addition to noise, leakage currents and interconnects with high capacitances will form a problem.
The energy burnt in computation accounts for 8% of global electricity use and ICT energy use is doubling every decade . ICT contributes as much to climate change as the aviation industry. Moore’s Law, which has kept ICT energy in check for 50 years, will end in the next decade. Additionally, the cost of designing and manufacturing smaller chips is astronomical. According to IBS estimates, while it would cost $400 million for 5 nm chips, 3 nm would cost $650 million.
A new study represents a significant advance in topological transistors and beyond-CMOS electronics. First time that the topological state in a topological insulator has been switched on and off using an electric field. Researchers proved this is possible at room temperature, which is necessary for any viable replacement to CMOS technology in everyday applications.
A new Monash review focuses on recent research in topological insulators and magnetic materials heterostructures. The intriguing interplay of magnetism and topology in such heterostructures can give rise to new phenomena such as quantum anomalous Hall insulators, axion insulators, and skyrmions. All of these are promising building blocks for low-power electronics in the future.
Topological materials for Beyond CMOS low power electronics
The discovery of topological materials whose properties remain intact even when an object is stretched, twisted or deformed, led to the awarding of the Nobel Prize in physics in 2016 to F. Duncan M. Haldane, and others. Their topological nature means these states are resistant to change, and so stable to temperature fluctuations and physical distortion — features that could make them useful in devices.
Topological materials, hold promise for a wide range of technological applications due to their exotic electronic properties. “In everyday life we are familiar with conducting materials, such as copper and insulating materials, such as plastic or glass. However there are also topological insulators with very peculiar properties,” says the Utrecht University professor. “These materials are insulating in the bulk, but current can flow along the edges. Furthermore, the conductivity is quantised and varies in discrete steps. This special property, of being both a conductor and an insulator, has had semiconductor researchers excited for computers that operate on ultra-low power, while also being much faster and more reliable.
ARC FLEET Approaches to low power electronics
ARC Centre of Excellence in Future Low-Energy Electronics Technologies (or FLEET) is a collaboration of physicists, electrical engineers, chemists and material scientists from seven Australian universities developing ultra-low energy electronics aimed at reducing energy use in information technology (IT). The Centre was funded in the September 2016 ARC Linkage round.
Running a computer will always consume some energy, but we are a long way (several orders of magnitude) away from computers that are as efficient as the laws of physics allow. Several recent advances give us hope for entirely new solutions to this problem via new materials and new concepts.
“One recent step forward in physics and materials science is being able to build and control materials that are only one or a few atoms thick. When a material forms such a thin layer, and the movement of electrons is confined to this sheet, it is possible for electricity to flow without resistance. There are a range of different materials that show this property (or might show it). Our research at the ARC Centre for Future Low-Energy Electronics Technologies (FLEET) is focused on studying these materials.”
FLEET three research themes are heavily enabled by novel atomically thin materials, including topological materials (Research Theme 1) and atomically thin semiconductors (as hosts for excitons in Research Theme 2, and for realising non-equilibrium topological phenomena in Research Theme 3).
The Branch of mathematics called “topology” tells us that donuts and coffee cups are equivalent because we can deform one into the other without cutting it, poking holes in it, or joining pieces together. It turns out that the strange rules that govern how electricity flows in thin layers can be understood in terms of topology. This insight was the focus of the 2016 Nobel Prize, and it’s driving an enormous amount of current research in physics and engineering.
The first FLEET approach to achieve electrical current flow with near-zero resistance is based on a paradigm shift in the understanding of condensed matter physics and materials science — the advent of topological insulators. Unlike conventional insulators, which do not conduct electricity, topological insulators conduct along their boundaries. The one-dimensional boundary of a two-dimensional (2D) topological insulator conducts electrons strictly in one direction, without the scattering that causes dissipation and heat in conventional materials. This one-way flow is due to the chiral nature of the current-carrying electrons.
This research theme will produce 2D topological insulators with large bandgaps in their interior, sufficient for conduction of edge modes at room temperature without dissipation. Ultra-low power topological transistors will be developed, in which the dissipationless channel along the boundary between a topological and conventional insulator is switched on and off by the application of a gate voltage.
Approaches being used within FLEET’s Research theme 1 to achieve dissipationless topological paths are: magnetic topological insulators and quantum anomalous Hall effect (QAHE), topological Dirac semimetals (including oxide ‘antiperovskites’) and artificial topological systems (artificial graphene and 2D topological insulators).
Additionally novel substrate materials with electric and magnetic ordering are needed to provide strong control of the properties of atomically thin materials. To provide these materials, FLEET will draw on extensive expertise in materials synthesis in Australia and internationally, from bulk crystals to thin films to atomically thin layer.
Topological material switched off and on for the first time, reported in Dec 2018
The Australian Research Council’s Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET) is realising new types of electronic conduction without resistance in solid-state systems at room temperature. These concepts will form the basis of new types of switching devices (transistors) with vastly lower energy consumption per computation than silicon CMOS. Electronic conduction without resistance will be realised in topological insulators that conduct only along their edges and semiconductors that support the superflow of electrons strongly coupled to photons. These pathways are enabled by the new science of atomically thin materials.”
Now, FLEET researchers at Monash University, Australia, have for the first time successfully ‘switched’ 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 — a proposed new generation of ultra-low energy electronic devices. Ultra-low energy electronics such as topological transistors would allow computing to continue to grow, without being limited by available energy as we near the end of achievable improvements in traditional, silicon-based electronics (a phenomenon known as the end of Moore’s Law).
“Ultra-low energy topological electronics are a potential answer to the increasing challenge of energy wasted in modern computing,” explains study author Professor Michael Fuhrer. “Information and Communications Technology (ICT) already consumes 8% of global electricity, and that’s doubling every decade.” This new study is a major advance towards that goal of a functioning topological transistor.
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,” explains 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). “This new switch works on a fundamentally different principle than the transistors in today’s computers,” explains Dr Edmonds. “We envision such switches facilitating a completely new computing technology, which uses lower energy.” The electric field induces a quantum transition from ‘topological’ insulator to conventional insulator.
To be a viable alternative to current, silicon-based technology (CMOS), 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.
(In this study, experiments were conducted at cryogenic temperatures, but the large band-gap measured confirms that the material will switch properly at room temperatures.)
Future Electronics Require a Combination of Topology and Magnetism
If suitable candidate materials are discovered, it is possible to realize these exotic states at room temperature and without the use of a magnetic field, assisting FLEET’s search for future low-energy, beyond-CMOS electronics.
A new review throws the spotlight on heterostructures of topological insulators and magnetic materials, where the interplay of magnetism and topology can give rise to exotic quantum phenomena that are promising building blocks for future low-power electronics.
“Our goal was to investigate promising new methods of achieving the quantum Hall effect,” says Dr Semonti Bhattacharyya of Monash University, who led the new study. The quantum Hall effect (QHE) is a topological phenomenon that allows high-speed electrons to flow at the edge of a material, which could be useful in future low-energy electronics and spintronics.
“However, the fact that quantum Hall effect always requires high magnetic fields, which are not possible without either high energy use or cryogenic cooling, is a severe bottleneck for this technology being useful.”
“There’s no point in developing ‘low energy’ electronics that require more energy to operate!” says Dr Bhattacharyya, a Research Fellow at FLEET who is looking for the next generation of low-energy electronics.
However, a ‘cocktail’ of topological physics and magnetism can produce a similar effect, the quantum anomalous Hall effect, in which similar edge states appear without the application of an external magnetic field.
To induce magnetism in topological insulators, several strategies have been used:
by incorporating magnetic impurity,
by using intrinsically magnetic topological insulators, and
by inducing magnetism through a proximity effect in topological insulator-magnetic insulator heterostructures.
“We focused on recent scientific research into heterostructures on the third approach in our review,” says co-author Dr Golrokh Akhgar (FLEET/Monash). In other words, a single structure with thin-film layers of topological insulators and magnetic materials adjacent to each other that allows the topological insulator to borrow magnetic properties from its neighbor.
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