High detectivity is essential for solar-blind deep-ultraviolet (DUV) light detection because the DUV signal is extremely weak in most applications. In this work, we report ultrahigh-detectivity AlGaN-based solar-blind heterojunction-field-effect phototransistors fabricated utilizing dual-float-photogating effect. The p+-Al0.4GaN layer and Al0.4GaN absorber layer deposited on the Al0.6GaN barrier serve as top pin-junction photogate, while the thin Al0.4GaN channel layer with a strong polarization field inside acts as virtual back photogate. Due to the effective depletion of the two-dimensional electron gas at the Al0.6Ga0.4N/Al0.4Ga0.6N heterointerface by the top photogate, the dark current was suppressed below 2 pA in the bias range of 0 to 10 V. A high photo-to-dark current ratio over 108 and an optical gain of 7.5×104 were demonstrated at a bias of 5 V. Theoretical analysis indicates that the optical gain can be attributed to the joint action of the floating top and back photogates on the channel current. As a result, a record high flicker noise (Johnson and shot noise) limited specific detectivity of 2.84×1015(2.91×1017)cmHz0.5W-1 was obtained. Furthermore, high response speed at the microsecond level was also shown in the devices. This work provides a promising and feasible approach for high-sensitivity DUV detection.