Heterojunction field-effect phototransistors using two-dimensional electron gas (2DEG) for carrier transport have great potential in photodetection owing to its large internal gain. A vital factor in this device architecture is the depletion and recovery of the 2DEG under darkness and illumination. This is usually achieved by adding an external gate, which not only increases the complexity of the fabrication and the electrical connection but also has difficulty ensuring low dark current (Idark). Herein, a quasi-pseudomorphic AlGaN heterostructure is proposed to realize the self-depletion and photorecovery of the 2DEG, in which both the barrier and the channel layers are compressively strained, making the piezoelectric and spontaneous polarization reverse, thus depleting the 2DEG and tilting the entire barrier and channel band to form two built-in photogates. The fabricated solar-blind phototransistors exhibit a very low Idark below 7.1×10-10mA/mm, a superhigh responsivity (R) of 2.9×109A/W, a record high detectivity (D*) of 4.5×1021 Jones, and an ultrafast response speed at the nanosecond level. The high performance is attributed to the efficient depletion and recovery of the full 2DEG channel by the two photogates, enabling direct detection of the sub-fW signal. This work provides a simple, effective, and easily integrated architecture for carrier control and supersensitive photodetection based on polarization semiconductors.