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Gallium oxide is a wide-bandgap oxide semiconductor, the monoclinic beta polymorph being the stable form and the one developed for power electronics and solar-blind ultraviolet detection. Its bandgap near 4.8 electronvolts gives a critical breakdown field several times that of silicon or silicon carbide, which translates into a much lower on-resistance for a given blocking voltage, and the transparency that follows from the same wide gap is what makes the material attractive for detectors that must ignore visible light. Large single crystals can be grown from the melt by methods established for other oxides, which distinguishes it from gallium nitride and is one reason the material moved quickly from laboratory to device work ^[1]. Progress has not been uniform across the polymorphs, and the metastable phases present different opportunities and different problems ^[2]. Field-effect transistors on single-crystal substrates were demonstrated early and set the baseline for the technology ^[3]. Heterojunctions with a p-type oxide, nickel oxide among them, address the lack of usable p-type conduction ^[4], and the wide gap has been exploited in electromechanical transducers for dual-modality detection ^[5]. The absence of shallow p-type doping and the low thermal conductivity of the material remain the main obstacles.