In this work, the MoS2 functionalized AlGaN/GaN high-electron mobility transistor (HEMT) was utilized for the first time to detect toxic mercury (Hg2+) ions under ultraviolet (UV) illumination. The AlGaN/GaN HEMT was fabricated on the sapphire substrate, and corresponding structural and electrical properties were investigated. Subsequently, the optimization of MoS2 concentration for device functionalization was carried out by performing sensing analysis of Hg2+ ions on three devices, and it was observed that the 20 mg/mL concentration of MoS2 is optimum for the detection of Hg2+ ions. Furthermore, the detection of Hg2+ ions was performed under UV exposure, where the developed sensor showed much-improved sensitivity of $548.07~\mu \text{A}$ /ppb compared with normal light. The comparative analysis indicates the increase of three orders of magnitude in sensitivity under UV irradiation, and it is the highest sensitivity ever observed by the AlGaN/GaN HEMT sensor for Hg2+ ion detection. Moreover, the sensor also exhibits the limit of detection (LoD) of 6.14 parts per trillion (ppt) that is much lower than the World Health Organization (WHO) standard limit for Hg2+ ions in drinking water. Due to the photoexcitation process, the generation of electron-hole pairs provides more binding sites on the MoS2 surface, which results in the ultrasensitive detection of the Hg2+ ions at trace and ultratrace levels. © 1963-2012 IEEE.