We report a demonstration of single-crystal gallium oxide (Ga2O3) metal-semiconductor field-effect transistors (MESFETs). A Sn-doped Ga2O3 layer was grown on a semi-insulating β-Ga2O3 (010) substrate by molecular-beam epitaxy. We fabricated a circular MESFET with a gate length of 4 μm and a source–drain spacing of 20 μm. The device showed an ideal transistor action represented by the drain current modulation due to the gate voltage (VGS) swing. A complete drain current pinch-off characteristic was also obtained for VGS < −20 V, and the three-terminal off-state breakdown voltage was over 250 V. A low drain leakage current of 3 μA at the off-state led to a high on/off drain current ratio of about 10 000. These device characteristics obtained at the early stage indicate the great potential of Ga2O3-based electrical devices for future power device applications.

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga 2 O 3 ) power devices. Ga 2 O 3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt-growth methods. These two features have drawn much attention to Ga 2 O 3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga 2 O 3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal-semiconductor field-effect transistors, metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes.

N-type Ga2O3 homoepitaxial thick films were grown on β-Ga2O3(010) substrates by ozone molecular beam epitaxy. The epitaxial growth rate was increased by more than ten times by changing from the (100) plane to the (010) plane. The carrier concentration of the epitaxial layers could be varied within the range of 1016–1019 cm-3 by changing the Sn doping concentration. Platinum Schottky barrier diodes (SBDs) on 1.4-µm-thick β-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 mΩ cm2, and a forward voltage of 1.7 V (at 200 A/cm2).

Single-crystal gallium oxide (Ga 2 O 3) metal-oxide-semiconductor field-effect transistors were fabricated on a semi-insulating b-Ga 2 O 3 (010) substrate. A Sn-doped n-Ga 2 O 3 channel layer was grown by molecular-beam epitaxy. Si-ion implantation doping was performed to source and drain electrode regions for obtaining low-resistance ohmic contacts. An Al 2 O 3 gate dielectric film formed by atomic layer deposition passivated the device surface and significantly reduced gate leakage. The device with a gate length of 2 lm showed effective gate modulation of the drain current with an extremely low off-state drain leakage of less than a few pA/mm, leading to a high drain current on/off ratio of over ten orders of magnitude. A three-terminal off-state breakdown voltage of 370 V was achieved. Stable transistor operation was sustained at temperatures up to 250 C. V

Gallium oxide (Ga2O3) is a strong contender for power electronic devices. The material possesses excellent properties such as a large bandgap of 4.7–4.9 eV for a high breakdown field of 8 MV cm−1. Low cost, high volume production of large single‐crystal β‐Ga2O3 substrates can be realized by melt‐growth methods commonly adopted in the industry. High‐quality n‐type Ga2O3 epitaxial thin films with controllable carrier densities were obtained by ozone molecular beam epitaxy (MBE). We fabricated Ga2O3 metal‐semiconductor field‐effect transistors (MESFETs) and Schottky barrier diodes (SBDs) from single‐crystal Ga2O3 substrates and MBE‐grown epitaxial wafers. The MESFETs delivered excellent device performance including an off‐state breakdown voltage (Vbr) of over 250 V, a low leakage current of only few μA mm−1, and a high drain current on/off ratio of about four orders of magnitude. The SBDs also showed good characteristics such as near‐unity ideality factors and high reverse Vbr. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh‐voltage power switching applications.

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We developed a donor doping technique for β-Ga2O3 by using Si-ion (Si+) implantation. For the implanted Ga2O3 substrates with Si+=1×1019–5×1019 cm-3, a high activation efficiency of above 60% was obtained after annealing in a nitrogen gas atmosphere at a relatively low temperature of 900–1000 °C. Annealed Ti/Au electrodes fabricated on the implanted Ga2O3 layers showed ohmic behavior. The Ga2O3 with Si+=5×1019 cm-3 showed the lowest specific contact resistance and resistivity obtained in this work of 4.6×10-6 Ω·cm2 and 1.4 mΩ·cm, respectively.