For this issue of the SAMCO Interview, we visited Professor Kuzuhara at Graduate School of Electrical and Electronics Engineering, University of Fukui to learn about his research in nitride compound semiconductor transistors.

What can you tell us about your research?

     The community we live in seems to have more problems than it did in the past--one of which is the depletion of energy resources. Japan has no natural resources to speak of, so energy is a problem we can't avoid. The question of how to generate energy--given the fact that the traditional resources will eventually run out--must be considered, and a technique for efficient energy usage must be developed. So, I started to think about how the semiconductors in my area of research could contribute to sustainable energy development, and I became involved in research on nitride semiconductor transistors for power conversion. During the time I worked for a telecommunications company, I developed high-speed transistors for cell phones by using gallium arsenide, or GaAs. Currently, I conduct my research mainly on power devices made of gallium nitride, or GaN, which resists heat, withstands high voltages and is appropriate for high-frequency operation. GaN power devices are superior to silicon power devices, which are currently used widely. Power devices based on GaN prevent the unnecessary energy consumption that accompanies daily life. Compared to the widespread use of cell phones, the use of GaN has a less adverse impact on the progress of civilization, so that we can fully pursue this technology without reservation. My goal is for us to achieve self-sufficiency in the generation of electrical power.

What motivated you to start your current research?

     I had studied high-speed transistors for use in telecommunications. Ever since the appearance of cell phones on the market, I had worked to reduce the size of the terminals. The use of cell phones eventually became widespread, growing at a rate that surpassed all expectations. I was very happy with this turn of events. However, the first cell phone I used weighed about a kilogram. It was too heavy for me to hold comfortably in one hand, so I'd place it on a desk during a call. It wasn't a cell phone per se, but rather, a phone that was portable. The battery would run out of power in the middle of a conversation. I had to recharge the battery once a day, just so that the terminal would be ready for a telephone call. With the subsequent arrival of the cell phone, we began to experience the pleasure of talking on the phone while walking about town. It was great to be able to make a call outdoors, and of course it saved time. In the 1990s, a terminal weighing less than 500g was introduced to the telecommunications market, but with the increasing demand for smaller cell phones, battery service life became a key challenge. I started research on transistors to fulfill that objective, and as a result I became involved in research on GaAs. Even in my university days, I had studied ways to make GaAs, and when I joined NEC I studied the application of the material to the field of telecommunications. What a long history of involvement with GaAs I've had! By the time I became employed at Fukui University, I had already studied GaN as a potential replacement for GaAs. In those days I expected it to be several decades before GaN technology would become commonplace, but I was determined to study it anyway. When I moved to University of Fukui, I was still interested in the possible application of GaN to telecommunications. Today, however, my main interest is the achievement of energy conservation.

Tell us about the prospects of your research.

     I hope to replace all silicon power devices for power converters with GaN power devices. The power sources of cars, PCs and air conditioners, which use inverters, as well as household electrical appliances such as refrigerators, are all subject to the generation of heat. This includes heat from heat sources in various locations, or the so-called parasitic resistance components, on top of the heat from the CPUs. Si semiconductors, in particular, are almost exclusively used in power circuits for voltage conversion, but the frequencies aren't helpful in reducing the parasitic resistance, meaning the resistance at switch-on. Another disadvantage of Si is that it's prone to breakage under high voltages. Si is excellent in that it rapidly performs the same calculation or processing under low voltage, and does so with minimal power consumption. However, it's disadvantageous in the field of power devices for converting high electrical power. In order to avoid breakage under high voltage, Si devices are arranged vertically in order to reduce the voltage per device. Unfortunately, this configuration increases the parasitic resistance. In other words, the heat will always be generated when high voltage is applied; such a configuration is not an efficient approach. This is where the dilemma occurs. Moreover, it's a wasteful consumption of energy to use cooling equipment to dissipate the heat. However, GaN solves this problem. As a semiconductor, GaN generates less heat under high voltage and operates stably despite the heat. Using GaN can help in reduction of wasteful heat in hybrid cars. In addition, GaN helps to reduce car's weight because it doesn't require as much cooling as silicon, since GaN can withstand higher temperatures than silicon. There is another possible application that can be benefited by GaN, that is, power adapters for laptop computers can be reduced in size and installed internally. Another key technique for the downsizing of adapters is to increase the frequency of the power source. It is said that the reactance components of an electric circuit can be downsized if the frequency is increased, and that they can be provided in ICs. Because GaN initially drew attention in the context of high-frequency operation and is used in cell phones, it has excellent high-frequency characteristics and is optimal for power conversion. Only GaN has both of these advantages over Si. Therefore, while various materials are good candidates for power devices, I hope that GaN power devices, which will be smaller in size and exhaust less heat than Si power devices, replace power electronics units at home in future.

What points do you keep in mind as part of your work?

     I believe that the research results should be practical, and I always bear in mind the need to achieve those results when I work on the research project. That's what I shared with my young students. If you conduct research in too relaxed a manner, you'll soon lose people's attention. I put heavy emphasis on delivering results that yield scientific milestones, so that my research can meet the needs of society. Although it might seem attractive to engage in research that will take a hundred years, I prefer research that will be useful to the community in the near future. Upon graduation from school, only a small number of researchers will be allowed involvement in hundred-year research endeavors. After joining companies, nearly all graduates will be assigned to work on product development. I hope my students will be aware of the essential work they're suited for among the engineering development applications most required by the community and companies, so that they can anticipate and prepare for emerging business opportunities. I always tell students to perform their research according to a timetable. They have to keep those milestones in mind within the context of a couple of years, including the master's program.

Can you give us some comments on Samco equipment?

     I use the "SAMCO RIE-200iP" dry etching system as the main processing equipment for GaN. So, I'm very grateful to you for performing maintenance and giving advice before it becomes necessary for us to ask for help. It gives us a feeling of security, because you don't simply leave once the equipment is installed. It takes a considerable amount of time to understand new equipment in the field of semiconductors. To maintain the equipment in the same peak operating condition as when it was first installed, it's extremely important to have support from the manufacturer, which can give advice based on delivery track record. SAMCO specializes in chemical compound semiconductors, such as GaN, which makes the company an important source of support for us. We believe that because SAMCO understands the direction in which checmical compound semiconductors should be headed, and aligns with us on that understanding, we conclude that SAMCO's equipment suits us very well.

Is there anything you'd like to say to the people of SAMCO?

     We hope you'll attach importance to business with universities. You'll make smaller profits doing business with universities, but they offer good word-of-mouth advertising along with the free exchange of information. Emerging researchers will use equipment in their universities before they join companies, and consequently they'll remember the names of the equipment manufacturers. In that respect, we think it's important to consider your university as a customer. I also think that universities should study the upcoming products manufactured by companies; hopefully, universities can afford to use the kinds of sophisticated equipment found in corporate laboratories. This is not a simple matter. We've been struggling just to provide minimum facilities. Few companies other than SAMCO will help us in that regard, so I'm grateful for the efforts your company makes.

Thank you very much for sharing your time with us today!