Ultrawide-bandgap gallium oxide (Ga₂O₃) holds immense potential for crucial applications such as solar-blind photonics and high-power electronics. Although several Ga₂O₃ polymorphs, i.e., α, β, γ, δ, ε, and κ phases, have been identified, the band alignments between these phases have been largely overlooked due to epitaxy challenges and inadvertent neglect. Despite having similar stoichiometry, heterojunctions involving different phases may exhibit band offsets. Here, β-Ga₂O₃/κ-Ga₂O₃-stacked "phase heterojunction" is demonstrated experimentally. This phase heterojunction has a sharp and well-defined interface, and subsequent measurements reveal an unbeknown type-II band alignment with significant valence/conduction band offsets of ≈0.65 eV/0.71 eV. This alignment is promising for self-powered deep ultraviolet (DUV) signal detection, necessitating an internal electric field near the junction and matching the absorption properties for effective electron-hole separation. The fabricated phase heterojunction photodetector displays a responsivity of three orders of magnitude higher at 17.8 mA W^-1, with improved response times (rise time ≈0.21 s, decay time ≈0.53 s) under DUV illumination and without external bias in comparison to the bare β-Ga₂O₃ and κ-Ga₂O₃ photodetectors, confirming the strong interfacial electrical field. This study provides profound insight into Ga₂O₃/Ga₂O₃ heterojunction interfaces with different polymorphs, allowing the use of phase heterojunctions to advance electronic device applications.

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