First Demonstration of Gallium Oxide (Ga₂O₃) Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)

- Japan Paves the Way for Practical Applications of Next-Generation Innovative Power Devices -

June 19, 2013

The National Institute of Information and Communications Technology (NICT, President: Dr. Masao Sakauchi), in collaboration with Tamura Corporation (Tamura, CEO: Naoki Tamura) and Koha Co., Ltd. (Koha, CEO: Yasuhiro Nakashima), has developed gallium oxide (Ga₂O₃) metal-oxide-semiconductor field-effect transistors (MOSFETs).

Ga₂O₃ is a promising new semiconductor material for high-breakdown and low-loss power devices. Its excellent material properties, such as wide bandgap, can reduce the cost and energy consumption of power conversion. Ga₂O₃ power devices can be fabricated on native melt-grown single-crystal substrates, leading to a great advantage of Ga₂O₃ over other widegap semiconductors such as SiC, GaN, and diamond for low-cost mass production. The Ga₂O₃ MOSFETs have a structure and characteristics applicable for actual use as is. We expect that the development of Ga₂O₃ power devices will not only alleviate global energy problems but also play an important role in the semiconductor industry in the near future.

Details of this achievement will be presented at the 2013 Device Research Conference (DRC2013), which will be held from June 24^th to 26^th at the University of Notre Dame, USA.

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 Organization (NEDO).

Background

The worldwide quest for innovative technologies beyond Si for efficient power switching has led to the development of high-breakdown and low-loss power devices using widegap semiconductors such as SiC and GaN. We envision that gallium oxide (Ga₂O₃) will be even more competitive than SiC and GaN for power devices because of its wider bandgap and other superior material properties. Another potential benefit of Ga₂O₃ is that native single-crystal substrates can be mass-produced by the melt-growth method. However, R&D on Ga₂O₃ electrical devices has lagged in spite of the material’s high potential.

New Achievements

We fabricated Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs), which are the most suitable FET structures for practical applications. Highlights of this development are novel technologies of ion implantation doping and gate dielectric deposition. The Ga₂O₃ MOSFETs exhibited excellent device characteristics including (1) a high on-state drain current (39 mA/mm), (2) a large off-state breakdown voltage of 370 V, (3) an extremely low leakage current (less than a few pA/mm, which was the lower detection limit of the measurement instrument), and (4) a high on/off drain current ratio of over 10 orders of magnitude. These excellent device characteristics will lead to a large reduction in power loss during switching operations and will be suitable for practical power devices.

Future Prospects

This development of MOSFETs using the new widegap semiconductor Ga₂O₃ will pave the way for new high-performance power devices in both high- and low-voltage applications, contributing not only directly to the global energy savings but also to the economy of Japan by opening up new opportunities in the semiconductor industry. We anticipate that R&D on Ga₂O₃ power devices will be of global interest.

Appendix

Fig. 1:(a) Prospective applications of power semiconductor transistors

(b) Benchmarking the ideal performance limits of Ga₂O₃ against other representative semiconductors

Figure 1(a) shows the prospects of Ga₂O₃ and other power semiconductors as predicted from their material properties. Ga₂O₃ power devices will initially be targeted toward high-voltage and high-power applications. As Ga₂O₃ devices become more established, their applications will broaden to those of SiC and GaN by virtue of their simple and low-cost substrate production.

The performance of power devices is characterized by their on-resistance and breakdown voltage. A tradeoff relationship exists between these two parameters, whereby enhancing the breakdown voltage of the devices to realize high-voltage utilization necessarily increases their on-resistance and hence power loss in the form of heat generation. The ideal performance limit of Ga₂O₃ power devices is benchmarked against those of other representative power devices in Fig. 1(b), according to which the bottom-right corner represents better device performance. We can see from this estimate that Ga₂O₃ transistors are less lossy (i.e. more energy-efficient) than Si and SiC transistors at a given breakdown voltage by ~3,000 and ~10 times, respectively. The reduction in power loss corresponds directly to the amount of energy saved. Therefore, Ga₂O₃ has better characteristics as a power device material than not only Si but also SiC and GaN.

Fig. 2: 2-inch-diameter single-crystal β-Ga2O3 substrate fabricated by the melt-growthmethod

Fig. 2: 2-inch-diameter single-crystal β-Ga₂O₃ substrate fabricated by the melt-growth method

Figure 2 shows a photograph of a 2-inch-diameter single-crystal β-Ga₂O₃ melt-grown substrate. The melt-growth method is a low-energy-consumption, low-cost way of making large substrates because it does not require a high-temperature and/or high-pressure environment and uses less source material. As the substrate production cost accounts for a large percentage (~60%) of the total production cost of widegap power devices (SiC, GaN, and diamond) for which a melt-grown substrate technology is unavailable, the ability to mass-produce large area single-crystal substrates from a melt is a huge cost advantage of Ga₂O₃ over other widegap semiconductors. We anticipate that a large Ga₂O₃ substrate (over 6~8 inches in diameter) will cost at least 10-100 times less than SiC and GaN substrates.

Fig. 3:(a) Cross-sectional schematic illustration and (b) optical micrograph of Ga2O3 MOSFET

Fig. 3:(a) Cross-sectional schematic illustration and (b) optical micrograph of Ga₂O₃ MOSFET

Figures 3(a) and (b) show a schematic cross-sectional illustration and an optical micrograph, respectively, of a Ga₂O₃ MOSFET fabricated using the new device process technologies of (1) selective-area ion implantation doping and (2) dielectric film deposition.

Fig. 4:(a) Output and (b) transfer characteristics of Ga2O3 MOSFET

Fig. 4:(a) Output and (b) transfer characteristics of Ga₂O₃ MOSFET

Figure 4(a) shows the output characteristics of the Ga₂O₃ MOSFET. Effective gate control over the drain current was evident. Furthermore, the off-state breakdown voltage was 370 V, a very high value considering the fact that the device consisted of a simple FET structure without incorporating any techniques to increase the breakdown voltage. As shown in Fig. 4(b), the off-state drain leakage current was very small (a few pA/mm, which was less than the lower limit of the measurement instrument), resulting in a high on/off drain current ratio of over ten orders of magnitude. The low leakage current in the off state directly leads to low power loss.

The combination of material properties superior to other semiconductors’, high quality single-crystal substrates, and novel process technologies developed in this work have led to the excellent devices characteristics of Ga₂O₃ MOSFETs. Despite a simple transistor structure, these devices have already satisfied the requirements for practical applications.