Ion-Sensitive Field Effect Transistor as a PH Sensor

In this work, both the static and dynamic behaviors as well as the signal read-out circuits of ISFETs were studied. The standard NMOS structure in conjunction with the insulator-electrolyte capacitor was used to model the ISFET under study. The site-binding theory was incorporated to describe the chemistry occurring at the insulator/electrolyte interface. The mechanism of the threshold voltage drift was further explored. We propose that to better understand the drift, both slow responding sites and hydration effects need to be considered. It was found that, to better simulate the voltage drift, two exponential terms had to be employed with one governing the initial drift and the other the long term drift. In addition, a low noise differential signal read-out circuit was designed and simulation was carried out using LTspice. The output voltage of the system changes from -2.1 V to 0.5 V when the pH of the electrolyte changes from 12 to 0.