Ultrawide bandgap (UWBG) semiconductors, which are defined in terms of a bandgap energy larger than those of SiC and GaN, are getting attention as emerging materials. Gallium oxide (Ga₂O₃) is one of the typical UWBG materials and has attractive material properties represented by an extremely large bandgap of about 4.5 eV [1]. Furthermore, availability of high-quality, large-area native wafers manufactured from bulk single crystals synthesized by melt growth methods is another important material feature of Ga₂O₃ [2]. In this talk, I will introduce our state-of-the-art Ga₂O₃ metal-oxide-semiconductor field-effect transistor (MOSFET) and Schottky barrier diode (SBD) technologies developed for applications to power and harsh environment electronics.

The first single-crystal Ga₂O₃ transistors by means of metal-semiconductor FETs were demonstrated in 2011 [3]. As a result of continuous efforts, we developed lateral depletion-mode (D-mode) Ga₂O₃ MOSFETs with a gate-connected field plate (FP) [4]. The devices showed a large off-state breakdown voltage of 755 V, a large drain current on/off ratio of over nine orders of magnitude, stable device operation at temperatures up to 300°C, and negligible DC–RF dispersion. A high gamma-ray tolerance was also demonstrated for the bulk Ga₂O₃ channel by virtue of weak radiation effects on their output current and threshold voltage.

Lateral enhancement-mode Ga₂O₃ MOSFETs enabling normally-off operation were fabricated by Si-ion implantation doping in an undoped Ga₂O₃ layer to form source/drain contacts and access regions [5]. The undoped Ga₂O₃ channel layer that was fully depleted at thermal equilibrium gave rise to positive threshold voltage. A decent on-state drain current of 1.4 mA/mm and a sufficiently large drain current on/off ratio of six orders of magnitude were also achieved.

Recently, our FET development took a step forward to vertical D-mode Ga₂O₃ MOSFETs. In the device structure, the source was electrically isolated from the drain by a current blocking layer formed by Mg-ion implantation except at an aperture opening for drain current conduction. Despite a large source–drain leakage current due to an imperfect function of the current blocking layer, the drain current modulation by applied gate bias was successfully demonstrated.

We have also been developing vertical Ga₂O₃ SBDs. Ga₂O₃ FP-SBDs were fabricated on n^--Ga₂O₃ drift layers grown by halide vapor phase epitaxy on n⁺-Ga₂O₃ (001) substrates [6]. The illustrative device with a net donor concentration of 1.8×10¹⁶ cm^-3 exhibited a specific on-resistance of 5.1 mΩ·cm² and an ideality factor of 1.05 at room temperature. Successful FP engineering resulted in a high breakdown voltage of 1076 V. Note that at the breakdown condition, the peak electric field in the Ga₂O₃ drift layer was estimated to be over 5 MV/cm, which was much larger than theoretical limits for SiC and GaN.

Steady and rapid progress has been made on developing building blocks such as bulk wafers, epitaxial films, and device process techniques for realization of Ga₂O₃ electronics. We fabricated and characterized relatively simple Ga₂O₃ MOSFET and SBD structures. Both types of developed devices revealed excellent characteristics and thus demonstrated great potential of Ga₂O₃. Research and development in the next decade is certainly crucial for the road to industrialization of Ga₂O₃ electronic devices.

This work was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO).

[1] T. Onuma, S. Saito, K. Sasaki, T. Masui, T. Yamaguchi, T. Honda, and M. Higashiwaki, Jpn. J. Appl. Phys. 54, 112601 (2015).

[2] A. Kuramata, K. Koshi, S. Watanabe, Y. Yamaoka, T. Masui, and S. Yamakoshi, Jpn. J. Appl. Phys. 55, 1202A2 (2016).

[3] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi, Appl. Phys. Lett. 100, 013504 (2012).

[4] M. H. Wong, K. Sasaki, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, IEEE Electron Device Lett. 37, 212 (2016).

[5] M. H. Wong, Y. Nakata, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, Appl. Phys. Express 10, 041101 (2017).

[6] K. Konishi, K. Goto, H. Murakami, Y. Kumagai, A. Kuramata, S. Yamakoshi, and M. Higashiwaki, Appl. Phys. Lett. 110, 103506 (2017).