We report sensitive and selective AlGaN/GaN High Electron Mobility Transistor (HEMT)-based sensor for Lead ion (Pb2+) detection. The gate region of the HEMT was functionalized by 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The response of the sensor is observed by monitoring drain to source current (\text{I} {\textsf {DS}}) for different concentrations of Pb2+ ions at a fixed drain to source voltage (\text{V} {\textsf {DS}} ). Our sensor reaches the lower detection limit of 0.018 ppb, which is much lower than the standard detection limit recommended by the World Health Organization (WHO) for drinking water. Furthermore, the sensor exhibited a rapid response time of 4 seconds and high sensitivity of 0.607μ \text{A} /ppb. Moreover, the selectivity analysis was performed and found that the sensor was highly selective towards Pb2+ ions. The change in electron concentration at 2-dimensional electron gas (2DEG) upon the capture of Pb2+ ions at gate region by DMTD, causes a change in the \text{I} {\textsf {DS}} , which showed excellent sensing response towards Pb2+ ions. The highly sensitive, selective, and rapid detection of Pb2+ ions paves the way for stable sensing performance based on DMTD functionalized AlGaN/GaN HEMT sensor. © 1980-2012 IEEE.