A Physics-Based (Verilog-A) Compact Model for DC, Quasi-Static Transient, Small-Signal, and Noise Analysis of MOSFET-Based pH Sensors

High-resolution pH measurement is important for biomedical, food, pharmaceutical industries as well as agricultural/environmental monitoring. Among the pH-meters, FET sensors (pH-FETs) offer several advantages, such as, higher sensitivity, lower cost, and smaller size. In this paper, we develop a physics-based (Verilog-A) compact model to simulate dc, quasi-static transient, small-signal, and noise performance of pH-FET sensors. The Verilog-A implementation would allow pH-FET integration with complex signal processing circuits, and prediction of the integrated performance. The model predicts that FET thermal noise dominates in subthreshold, while FET flicker noise dominates above threshold. The minimum pH resolution is dictated by an interplay of FET and electrolyte noise sources, which are in turn functions of transistor geometry and operating condition. For a given technology node, if the sensor is designed to operate in subthreshold regime, pH-FET with shorter but wider channel has better pH resolution. In contrast, for sensor designed to operate in the inversion regime, longer and wider channel is preferred. Finally, we demonstrate the utility of the Verilog-A implementation by circuit simulation of a low-power sensor interface.