Design-Oriented Physical Model of Ion-Sensitive Field Effect Transistor for pH Sensing

In this work, we present a circuit design-oriented physical model of the ion-sensitive field-effect transistor (ISFET) for pH-sensing applications. Particularly, this model considers the site-binding interactions at the interface of the electrolyte and the insulator and utilizes a charge-based approach to analyze the field-effect device. The light numerical solution based on the Newton-Raphson-Jacobian method is obtained and validated with experimental results. A semianalytical Verilog-A model is developed by proper selection of initial value and truncating the number of iterations in the numerical method. It follows the numerical results in all regions of operation with a relative error of 2% and a much faster simulation time. The model has a few calibration parameters extracted to fit two experimental devices, including a p-type silicon-on-insulator (SOI) nanowire and conventional n-type ISFET devices. These results suggest that the proposed model is a promising candidate for the circuit-level simulation of ISFET.