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Achieving an Ultra-Low Power Loss Society through the Development of Next-Generation Power Devices
Research Institute of Green Device
In recent times, procedures and activities that are earth-friendly with minimal environmental impact have often been labeled with the adjective green, such as in the terms green technology, green IT, green management, and green consumption. The development of various types of technology and knowledge, as well as products and lifestyles, which can contribute to reducing the emission of greenhouse gases beginning with carbon dioxide (CO2) while improving the efficiency of energy production and consumption, is becoming an important social and economic issue to support the society of our future.
Professor Kawarada, acting as Director of the Research Institute of Green Device
One such technological field is that of green devices. Green devices, in short, encompass a wide range of fields. Areas of particular importance include the research and development of semiconductor materials, transistors, and displays necessary for green IT that is able to attain a type of information society with low power consumption, and the research and development of power devices that can bring about improvements in the efficiency of all processes related to the production, consumption, and storage of electric power, ranging from industrial fields to the everyday life of all people.
Under such circumstances of the times, Waseda University’s Research Institute of Green Device, established in 2010, engages in research activities primarily focused on power devices while also including green IT in its scope of investigation. Hiroshi Kawarada, Professor on the Faculty of Science and Engineering at Waseda and also Director of this project, which was selected for Waseda’s University Research Initiatives, spoke on the project’s strategies and vision.
Next-Generation Devices Created from Diamonds
The Research Institute of Green Device was originally created for participation in the Ministry of Education, Culture, Sports, Science and Technology’s Developing a Research Infrastructure Network for the Construction of a Low-Carbon Society program (NIMS official page) (2009 fiscal year supplementary budget). This program, which selected 15 universities and 3 research institutions from across the country incorporating the most influential seeds of green device research, was started with the objective of efficiently and effectively accelerating the innovation of such research at a national level. From Waseda University, Professor Kawarada’s research group participated with the theme of Research and Development of Ultra-Low Loss Power Transistors. In addition, since the 2010 fiscal year, the program was adopted as the Advanced Low Carbon Technology Research and Development Project (ALCA), which is a part of the Japan Science and Technology Agency’s (JST) Strategic Basic Research Programs, and the research topic of Fundamental Green Inverter Technology with Large-Diameter Diamond Substrates is being driven forward.
“As part of this program, which is based on the 3 core principles of production, conservation, and storage of energy, we have narrowed down our targets to research and development of next-generation power devices and worked on research projects with the goal of significant conservation of electrical power. Numerous losses occur within the processes for provision and consumption of electrical power, and therefore there is a need for next-generation devices that can make dramatic reduction of such losses possible.” (Professor Kawarada)
Figure 1: Achievement of streamlining and ultra-low on-resistance of power devices (vertical field effect transistors) by using next-generation materials
Traditional silicon semiconductors (left) required a thick drift layer of 100 µm or more, while with next-generation materials (right) such as silicon carbide (SiC), gallium nitride (GaN), and diamond, a reduction of electrical power loss is made possible as a result of lowered resistance and lowered parasitic capacitance due to a greatly diminished thickness of 5 µm or less.
Photo: Portion of experimental equipment for diamond devices (Kawarada Research Laboratory)
Top: Diamond deposition equipment using microwave plasma / Bottom: molecular beam epitaxial equipment for formation of nitride semiconductors on diamond)
In the semiconductor field, the practical realization of next-generation materials that can surpass the limits of traditional silicon materials is becoming accelerated. When high-voltage electric current flows through a semiconductor circuit (conducting state, on-state), power losses are generated due to the device resistance (on-resistance), and power losses occur during the time of switching from on (conducting) to off (interrupted) as well. In order to operate devices while avoiding insulation breakdown, a given amount of open space within the devices is required. However, if next-generation materials capable of yielding more stable conductivity with excellent high-voltage resistance can be developed, and devices can be streamlined by reducing the size of the required spaces, the on-resistance during switching would be greatly reduced, in turn lowering the conduction loss resulting from the on-resistance. At the same time, the parasitic capacitance and switching loss would be reduced as well. (Figure 1)
Silicon carbide (SiC), which has attracted much attention as a next-generation material, has been the subject of technological development across the globe, and is already in mass production. Diamond, however, is a material considered to be even more promising, which will rise to prominence following SiC. In 2012, Professor Kawarada and his associates succeeded in developing the world’s first diamond field-effect transistor (FET) that could operate at the high temperature of 400 degrees Celsius. By synthesizing artificial diamonds using nano-level technology and forming a conductivity region covered by a hydrogen atom layer with a 10 nm-level thinness on their surfaces, they were able to create a transistor capable of high-speed operation (Figure 2). Initially, there were only 2 or 3 locations, including Professor Kawarada’s group, where full-scale research into this practical realization was conducted, but presently the number of groups entering into such research is growing worldwide.
“By covering the diamond surface with hydrogen atoms, a positive electric charge sufficient to operate the transistor appears at the surface, and only the surface becomes a low-resistance p-type semiconductor(*). We paid particular attention to this phenomenon while proceeding with our practical development.” (Professor Kawarada). (*Note: Semiconductors consist of p-type semiconductors, which use holes that act as carriers of electric charge; and n-type semiconductors, which use free electrons that hold negative electric charge)
Figure 2: Operating principles of diamond transistors
Achieving Next-Generation Inverters with Ultra-Low Loss
A key device showing positive results for conservation of electrical power consumption is the inverter. Inverters are electrical circuits that efficiently convert DC voltage to AC voltage of a desired frequency, and are widely used in power systems, with familiar applications including air conditioners, refrigerators, subways, and bullet trains, as well as extending to recent use in hybrid cars, electric vehicles, and industrial robots. The AC frequency can be directly linked to a motor’s rotation speed, for continuous and smooth startup and acceleration of automobiles and trains. Even in non-power system applications such as solar cells and storage batteries, they can efficiently convert DC power to AC power for electrical power transmission. The introduction and evolution of inverters have been important elements in the improvement of energy conservation up until now, but by substituting diamond semiconductors for power devices, electrical power losses can be dramatically reduced even further (Figure 3).
“The spread of next-generation inverters using diamond semiconductors will enable conservation of enormous amounts of power consumption. There are even preliminary calculations predicting a level of power conservation on the scale of 5 to 6 nuclear power plants.” (Professor Kawarada)
Figure 3: Comparison of inverter losses at 1000V high-voltage operation
(Calculation uses 5 million automobiles with the assumption of high-voltage operation such as hybrid cars and electric vehicles x 500 hours yearly travel time)
In addition, expectations are also high for applications of power regeneration brakes that can convert the kinetic energy of resistance applied to brake systems, such as those for automobiles, railways, and elevators, into electrical energy that they can use again. Because this technology would allow motors to not only be used as drive systems but also as power generation systems, it involves not only conservation of energy, but also its production. The range of speed at which such regeneration brakes can be used depends on the heat resistance of power devices. Although silicon devices can only be used for brake operation at 35-40 km/h or lower, silicon carbide, which is able to withstand higher temperatures, allows use at speeds of up to around 70 km/h, and is already being utilized in part of the Ginza Line of the Tokyo Metro subway.
“High-temperature operation enabled by the practical realization of silicon carbide is said to be possible at temperatures of around 250 degrees Celsius. Using diamond devices that can withstand temperatures of 400 degrees Celsius and higher, we are currently working on the further development of high-temperature operation circuits for engine-related components, nuclear reactors, and surveillance satellites. Regeneration brakes usable at even higher speeds are not out of reach.” (Professor Kawarada)
Toward Comprehensive Green Device Research
Although initial development was started with a narrowed target of ultra-low loss power devices, the current scope of research continues to expand more and more, into fields such as application research into diamond device MEMS, and investigations into improvements in the efficiency of solar cells and organic ELs through the creation of p-type transparent electrodes that had never existed before (Figure 4).
“If semiconductor integrated circuits (LSI) can be considered as the ‘brain’ governing an information society, then power devices can be seen as its ‘heart’, controlling the circulation of the society’s energy. In the times to come, the Research Institute of Green Device will devote its efforts to production and storage of energy in addition to its conservation, and we plan to widen the scope of our research while fully applying the strengths of this university.” (Professor Kawarada)
There is great anticipation for the development of this research institute, with the powerful key technology of diamond devices at its core and with its large contributions to the realization of a green society through the gathering of diverse green device technology.
Figure 4: Improvement in efficiency of solar cells through the realization of p-type transparent electrodes
Solar cells and organic ELs require transparent electrodes that allow the passage of light. Up until now, n-type transparent electrodes have existed, but this marks the world’s first p-type transparent electrode. By joining the n-type and p-type on both sides as shown in the Figure, the power production efficiency of solar cells can be greatly increased.
Waseda University Research Institute of Green Device
Waseda University Organization for University Research Initiatives
Waseda University Faculty of Science and Engineering
Ministry of Education, Culture, Sports, Science and Technology Developing a Research Infrastructure Network for the Construction of a Low-Carbon Society program (NIMS official page)
Japan Science and Technology Agency (JST) Strategic Basic Research Programs, Advanced Low Carbon Technology Research and Development Project (ALCA)