A low-hysteresis voltage and high-sensing linearity chloride ionsensitive 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 EGFETbased chloride ion-sensitive microsensors having same channel width/ length ratio (1000 μm∕10 μm) but with different channel geometries (rectangular and annular types) are presented. At pCl 3 (log½Cl − ¼ −3) test point, a very small hysteresis voltage of the rectangular-and annularchannels EGFET-based Cl − microsensors (5 and 7 mV, respectively) can be achieved. As the concentrations tested ranging from pCl 1 (log½Cl − ¼ −1) to pCl 5 (log½Cl − ¼ −5), a very high-sensing linearity (99.23% and 99.08%) of the two types of EGFET-based Cl − microsensors is achieved. However, the sensitivity of the rectangular-channel EGFETbased Cl − microsensor (45 mV∕pCl) is much higher than that of the annular-channel EGFET-based Cl − microsensor (37 mV∕pCl). The selectivity coefficient of the investigated EGFET-chloride ion sensor under four different interfering ions (OH − , F − , SO 2− 4 , and Br − ) are also measured and analyzed.
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