Gallium oxide (Ga₂O₃) has excellent material properties for power device applications represented by the extremely large breakdown field due to a large bandgap of 4.7-4.9 eV. Another important feature in industry is that large single-crystal β-Ga₂O₃ bulks and wafers can be fabricated with melt-growth methods. We recently succeeded in fabricating Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs) by using newly developed technologies for making single-crystal bulks, growing conductivity-controlled epitaxial films, and fabricating devices. The MOSFETs exhibited excellent device characteristics including an off-state breakdown voltage over 370 V, an extremely low leakage current, and a high on/off drain current ratio of more than ten orders of magnitude. These results indicate that Ga₂O₃ have comparable or even more potential than Si and typical widegap semiconductors SiC and GaN for power device applications.