Understanding the Extended-gate FET pH Sensor Sensing Mechanism through Equivalent Circuit Simulation in LTSpice

This paper presented the design and simulation of an electrochemical equivalent circuit for a pH sensor based on an extended-gate field-effect transistor (EGFET). The pH sensor is critical to many domains, including industrial operations, environmental monitoring, and biomedical applications. To examine the equivalent circuits used in an EGFET sensor for pH measurement, specifically focusing on an electrochemical sensing response and further comprehend the EGFET sensor's sensing mechanism, multiple equivalent circuits were built using resistors and capacitors at the FET's gate to replicate the sensing electrode (SE), reference electrode (RE), and pH solution. The circuit configurations and discrete component values were then changed and simulated with the LTSpice XVII program in order to examine and understand the behavior of the sensing reaction. The circuit configurations were considered the sensor's input, and changes in the metal oxide semiconductor field effect transistor (MOSFET) drain current (ID) corresponding to the various circuit configurations and component values were recorded and examined. It was discovered that the capacitor(s) linked directly to the MOSFET gate are the primary factor that affects ID, whereas resistors did not affect ID. This observation provides empirical evidence supporting the hypothesized electrochemical events taking place on the surface of the SE, hence elucidating the significance of ionic exchange. Ions play a role in the capacitor mechanism, which is associated with charges.