The p-GaN gate high electron mobility transistor (HEMT) with a 2.0 × 10^-5 cm² sensitive area as a UV photodetector (PD) has been designed and fabricated in this study. AlGaN/gallium nitride (GaN) heterostructure was adopted to get a 2-D electron gas (2DEG) as a conductive channel, resulting in a high photoresponsivity of 8.07 × 10⁴ A/W, a sharp cutoff wavelength at 360 nm, high UV-to-visible rejection ratio of 1.80 × 10⁶, rise and decay time of 0.12 and 1.0 ms, respectively. The dark current of 5.44 × 10^-7 A, the photocurrent of 4.42 × 10^-3 A at 5 V, the external quantum efficiency (EQE) of 2.77% × 10⁵%, and the detectivity of 8.31 × 10¹⁴ Jones for the UV PD were determined under a low UV light intensity of 5 mW/cm² at 360-nm UV illumination. The obtained results show that the performance of as-fabricated UV PD based on AlGaN/GaN HEMT is significantly improved compared to the literature. This device, which has lower noise equivalent power (NEP) and enhanced detectivity features compared to existing ones, is a promising candidate for military and space applications.