Bare Au-gated and thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect mercury(II) ions. Fast detection of less than 5 seconds was achieved for thioglycolic acid functionalized sensors. This is the shortest response time ever reported for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN/GaN HEMT based sensors showed 2.5 times larger response than bare Au-gated based sensors. The sensors were able to detect mercury (II) ion concentration as low as 10-7 M. The sensors showed an excellent sensing selectivity of more than 100 for detecting mercury ions over sodium or magnesium ions. The dimensions of the active area of the sensor and the entire sensor chip are 50 m 50 m and 1 mm 5 mm, respectively. Therefore, portable, fast response, and wireless based heavy metal ion detectors can be realized with AlGaN/GaN HEMT based sensors.