Ultraviolet (UV) photodetectors (PDs) have attracted significant attention for civil and military applications. Zinc oxide (ZnO) and nickel oxide (NiO) have been widely applied in UV-PDs due to their wide bandgap (3.2–3.8 eV), transparency, excellent optical and electrical properties, and good chemical stability. However, UV signals are generally weak; hence, UV-PDs with high optical gain are essential. In this work, high-performance solar-blind p-NiO/n-ZnO/p-Si heterojunction bipolar phototransistors (HBPTs) were fabricated. The fabricated HBPTs exhibited a high responsivity of $9.4\times 10^{{3}}$ A/W at a wavelength of 280 nm with ${V}_{\text {CE}}$ = −7 V, a high optical gain of $3.96\times 10^{{4}}$ , and a large detectivity of $3\times 10^{{13}}$ Jones. In addition, the UV/visible rejection ratio was as high as 880. These behaviors indicate that the prepared HBPTs are good solar-blind PDs and suitable for the detection of weak UV signals. However, for ${V}_{\text {CE}}$ value below −7 V, ( $\vert {V}_{\text {CE}}\vert >$ 7 V), the optical gain decreased due to the punchthrough effect. Furthermore, band diagrams showed that the photogenerated electrons were blocked by the potential barrier at the NiO/ZnO interface (base–emitter junction) due to a large conduction-band discontinuity ( $\Delta {E}_{C}{)}$ of 2.7 eV, which resulted in a large optical gain in the prepared HBPTs.