Overcoming solar-blind limitations: 3DEG-induced Fermi-level control in AlGaN phototransistors enables 10 out-of-band rejection and 650-photons/s detection.

Solar-blind ultraviolet (SBUV) photodetectors are critical for applications requiring immunity to solar background noise, such as flame sensing and corona discharge detection. Among these, AlGaN-based heterojunction field-effect phototransistors (HFEPTs) stand out for their high sensitivity enabled by high-conductivity two-dimensional electron gas (2DEG) channels. However, conventional HFEPTs suffer from severe out-of-band photoresponse and persistent photoconductivity (PPC) decay due to deep-level heterojunction defects, limiting their practical utility in sunlight-rich environments. To address this, we propose a 3DEG-engineered AlGaN phototransistor that leverages a composition-graded channel to modulate the Fermi level. This design achieves an unprecedented spectral rejection ratio of 10(260/400 nm)-surpassing 2DEG-based devices by five orders of magnitude-while maintaining sub-fW sensitivity (0.6 fW/cm, equivalent to 650 photons/s). Band structure analysis reveals that the Fermi-level pinning mechanism prevents deep-acceptor-level transitions under both dark and illuminated conditions, effectively suppressing the parasitic out-of-band response and accelerating PPC decay by six orders of magnitude within 10 s. With a specific detectivity (*) of 5.1×10 Jones, the 3DEG-PT advances accurate solar-blind operation by detecting weak UV signals under solar irradiation. Our Fermi-level engineering strategy mitigates deep-level impacts, offering a scalable approach for UV photodetectors requiring high background rejection.