Novel Crystal Technology (NCT), a global leader in Gallium Oxide (Ga₂O₃) technology, has announced the successful development of a Gallium Oxide vertical MOS transistor (β-Ga₂O₃ MOSFET) with 1.23 GW/cm² of the Power Figure of Merit (PFOM). This PFOM is the world’s highest value for β-Ga₂O₃ FETs and is 3.2 times higher than the previous highest value published by other research institutes. Part of this R&D effort was subsidized under the “Research and Development of Inversion-type channel Gallium Oxide MOS Transistors” program of the National Security Technology Research Promotion Fund (JP004596) of the Acquisition, Technology and Logistics Agency (ATLA) in Japan.

The results of this development will greatly advance the development of medium- to high-voltage (0.6–10 kV) gallium oxide transistors, which will lead to lower prices and higher performance of power electronics. In the future, the improvement of the efficiency and footprint of power electronic equipment such as industrial inverters and power supplies is expected to contribute to the electrification of the world, for instance electric vehicles, HVDC, and renewable energies such as solar and wind power which connect power systems to the grid. Details of these results was presented at the 72nd JSAP Spring Meeting on March 15, 2025, “Demonstration of β-Ga₂O₃ FinFET with power figure of merit of 1.23 GW/cm² “.

Overview

Wide Bandgap materials, for example, Silicon Carbide (SiC) and Gallium Nitride (GaN) are under development to replace Silicon (Si) power devices. Gallium Oxide (β-Ga₂O₃) has even superior material properties, and cost is more competitive due to faster crystal growth speed. It is expected to be applied to various power electronic devices such as home appliances, industrial equipment, electric vehicles, trains, solar power generation, and wind power generation, because of its low-loss and low-cost characteristics. Therefore, R&D around Ga₂O₃ is accelerated at companies and research institutes in the world, to reduce the size and efficiency of electrical equipment.

NCT has been working on commercialization of β-Ga₂O₃ MOSFETs under the “Research and Development of 10 kV Class Gallium Oxide Trench MOSFETs” and “Research and Development of Inversion-type channel Gallium Oxide MOS Transistors” programs, since awarded the ATLA contract in 2019. This time, the world’s highest PFOM^*5 for β-Ga₂O₃ MOSFETs, 1.23 GW/cm² was achieved by incorporating a high-resistance guard ring structure with Mg ion implantation at the edge of gate electrode in a drift layer. The accomplishment will greatly advance the development of medium- to high-voltage (0.6–10 kV) Gallium Oxide transistors, which will lead to lower prices and higher performance of power electronics.

Achievements

Although NCT has been developing β-Ga₂O₃ MOSFETs, material’s superior high breakdown electric field strength (6–8 MV/cm) has not been fully exploited, due to the high electric field at the edge of the gate electrode. In other materials, a p-type conductive layer is used at the edge of the electrode to reduce the electric field, but for Gallium Oxide, the p-type conductive layer technology has not yet been established, and the same method could not be applied. Therefore, instead of the technically unestablished p-type conductive layer, NCT’s researchers used a high-resistance β-Ga₂O₃ layer for the guard ring, and Mg, an acceptor impurity that forms a deep level, was added to the layer through ion implantation and activation heat treatment processes.

Figure 1 shows the cross-sectional structure and plan view of the newly developed β-Ga₂O₃ MOSFET.

Key Features of the Newly Developed Transistor

• Vertical device structure that is advantageous for low loss and high current required for power devices
• Multi-fin structure with sub-micron mesa width channel that enable normally-off operation without a p-type conductive layer
• β-Ga₂O₃ drift layer with a low donor concentration of 7.5×10¹⁵ cm^-3, and a thick thickness of 55 μm.
• Guard-ring structure with Mg ion implantation to β-Ga₂O₃ in the area around the gate edge to reduce the electric field at the region.

The prototype β-Ga₂O₃ MOSFET has 10 fins with a mesa width of 0.2 μm, a gate length of 3.5 μm, a fin length of 70 μm, and a fin pitch of 5 μm.

Figure 2 shows the drain current-voltage characteristics of a prototype β-Ga₂O₃ MOSFET. It exhibits normally off characteristics in which no current flows at gate voltage of 0 V. The maximum current density normalized by the source electrode area (50 μm × 60 μm) is 218 A/cm², and the specific on-resistance is 21.6 mΩ·cm² (V_gs = 5 V).

Figure 3 shows the dependence of the drain and gate currents on the gate voltage. The drain current on/off ratio is as large as 8 orders of magnitude or more, and the subthreshold slope is 162 mV/decade, resulting in good transistor characteristics.

Figure 4 shows the source and gate current characteristics when the gate voltage and source voltage are fixed at 0 V and a positive voltage is applied to the drain electrode. By applying a guard ring structure with Mg ion implantation, the breakdown voltage was increased from 1.6 kV to 5.15 kV. The maximum electric field strength in the β-Ga₂O₃ drift layer before and after the application of the guard ring structure is estimated to be 2 MV/cm and 3.72 MV/cm, respectively. NCT believes that the Mg guard ring reduces the concentration of the electric field at the gate electrode end and makes it possible to increase the electric field strength in the drift layer.

Figure 5 shows the relationship between the specific on-resistance (R_{on, sp}) and breakdown voltage (V_br), which is a performance indicator of power devices. The breakdown voltage was greatly improved by incorporating a guard ring structure formed by ion implantation of Mg into β-Ga₂O₃ in the electrode termination region. As a result, it is 13.3 times more than before the application of the guard ring, a great PFOM value of 1.23 GW/cm² was obtained. This PFOM value is the highest in the world for β-Ga₂O₃ FETs and is 3.2 times higher than the previous highest value for β-Ga₂O₃ FETs published by other research institutes.

What’s Next

The termination structure of β-Ga₂O₃ MOSFETs is further enhanced by utilizing hetero p-type semiconductor materials such as NiO, etc., to further reduce the electric field concentration at the electrode termination. NCT’s researchers aim to realize a β-Ga₂O₃ power transistor with performance exceeding that of SiC’s by exploiting the potential of β-Ga₂O₃‘s high breakdown field strength (6–8 MV/cm) with an advanced termination structure.

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