We have designed a metal-semiconductor-metal (MSM) solar-blind ultraviolet (UV) photodetector (PD) by utilizing Al0.55Ga0.45N/Al0.4Ga0.6N/Al0.65Ga0.35N heterostructures. The interdigital Ni/Au metal stack is deposited on the Al0.55Ga0.45N layer to form Schottky contacts. The AlGaN hetero-epilayers with varying Al content contribute to the formation of a two-dimensional electron gas (2DEG) conduction channel and the enhancement of the built-in electric field in the Al0.4Ga0.6N absorption layer. This strong electric field facilitates the efficient separation of photogenerated electron-hole pairs. Consequently, the fabricated PD exhibits an ultra-low dark current of 1.6 × 10-11 A and a broad spectral response ranging from 220 to 280 nm, with a peak responsivity of 14.08 A/W at -20 V. Besides, the PD demonstrates an ultrahigh detectivity of 2.28 × 1013 Jones at -5 V. Furthermore, to investigate the underlying physical mechanism of the designed solar-blind UV PD, we have conducted comprehensive two-dimensional device simulations.