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A new candidate for next-generation power devices

Gallium oxide transistors could rival silicon carbide and gallium nitride in power device applications ; they could be cheaper and save more energy when produced on mass

The National Institute of Information and Communications Technology (NICT) says it has helped to develop and test the world's first single-crystal gallium oxide (Ga2O3) field-effect transistors. The research is a collaboration between NICT and Japan-based firms, Koha Co. and Tamura Corporation. Ga2O3 is a promising new semiconductor material for high-breakdown and low-loss power devices because of its excellent material properties, such as a wide bandgap. However, research & development on Ga2O3 electrical devices has lagged in spite of this. Hence NICT and its collaborators decided to look in to the potential of the material. Their latest research estimates that as Ga2O3 devices become more popular, their applications will broaden to those of SiC and GaN due to the possibility of easy and low-cost substrate production (see Figure 1(a)) .      Fig. 1: (a) Applications of semiconductor transistors in the near future; (b) Relationship between on-resistance and breakdown voltage of representative semiconductors and Ga2O3 for power devices Figure 1(b) shows that the material has better power device performance toward the bottom-right corner of the graph. There is a trade-off between "breakdown voltage" and "on-resistance", which together determine the power device performance. The straight lines in Fig. 1(b) show the ideal performance limits of the materials. It is estimated that Ga2O3 has better characteristics as a power device material than not only Si but also SiC and GaN. Another advantage of a Ga2O3 substrates is that they can be fabricated in the melt. The melt-grown method is a low-energy-consumption, low-cost way to make large wafers because it does not need a high-temperature and/or high-pressure environment and its material efficiency is high. The method is especially suitable for mass production. The scientists say the ability to fabricate single-crystal substrates is a big advantage of Ga2O3 for industry because other widegap semiconductors (SiC, GaN, and diamond) cannot be made in this way.  Fig. 2: Two-inch square single-crystal β-Ga2O3 substrate fabricated by the melt-grown method The latest results were obtained by fabricating a Ga2O3 metal-semiconductor field-effect transistor (MESFET) by using a high-quality n-type Ga2O3 thin film on a single-crystal Ga2O3 substrate grown by MBE as a channel layer of the MESFET. Figures 3(a) and (b) show a schematic cross-sectional illustration and a micrograph of a fabricated Ga2O3 MESFET. The scientists used newly developed Ga2O3 single-crystal substrate, epitaxial thin film growth, and device-process techniques to fabricate the device.     Fig. 3 : (a) Cross-sectional schematic illustration and (b) optical microscope micrograph of Ga2O3 MESFET Figure 4 shows the output and transfer characteristics of the Ga2O3 MESFET. Complete on/off control of the drain current was achieved by using a gate voltage swing. The Ga2O3 MESFET exhibited an off-state breakdown voltage over 250 V, a very high value considering that the device has a simple FET structure without any measures to increase the breakdown voltage (Figure 4(a)).     Fig. 4 : (a) Output and (b) transfer characteristics of Ga2O3 MESFET As shown in Fig. 4(b), the off-state drain leakage current is very small (a few μA), resulting in a high on/off drain current ratio of about 10,000. The low leakage current at the off state directly leads to a decrease in power loss. These device characteristics obtained at the early stage of development indicate the great potential of Ga2O3 -based electrical devices for future power-device applications. The scientists hope the latest development of transistors using Ga2O3 will pave the way for new high-performance power devices. They expect that R&D on Ga2O3 power devices will take off and that future devices will be used in many different applications. NICT expects that Ga2O3 power devices will not only contribute to solving global energy problems but also play an important role in the semiconductor industry in the near future. Details of this achievement were published online in the paper "Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal β- Ga2O3 3 (010) substrates", by M, Higashiwaki et al, Applied Physics Letters, vol. 100, pp. 013504 (2012). DOI: 10.1063/1.3674287 This work was partially supported by "The research and development project for innovation technique of energy conservation" of the New Energy and Industrial Technology Development Organisation (NEDO).

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