A low-hysteresis voltage and high-sensing linearity chloride ion-sensitive sensor based on an extended gate field-effect transistor (EGFET) for real-time water quality monitoring microsystem applications is presented. All of the EGFET-manufacturing processes adopted in this work are compatible with standard integrated circuits planar technology, and therefore, they are very suitable for the mass production. Two EGFET-based chloride ion-sensitive microsensors having same channel width/length ratio () but with different channel geometries (rectangular and annular types) are presented. At pCl 3 () test point, a very small hysteresis voltage of the rectangular- and annular-channels EGFET-based microsensors (5 and 7 mV, respectively) can be achieved. As the concentrations tested ranging from pCl 1 () to pCl 5 (), a very high-sensing linearity (99.23% and 99.08%) of the two types of EGFET-based microsensors is achieved. However, the sensitivity of the rectangular-channel EGFET-based microsensor () is much higher than that of the annular-channel EGFET-based microsensor (). The selectivity coefficient of the investigated EGFET-chloride ion sensor under four different interfering ions (, , , and ) are also measured and analyzed.