A behavioral macromodel of the ISFET in SPICE

Physico-chemical models of the ISFET (Ion-Sensitive Field-Effect Transistor) were developed by the authors in the past, as SPICE built-in models (BIOSPICE). This approach has some drawbacks, i.e., the need of availability of the program source, a deep knowledge of the code subroutines and structure, and the need of compiling the whole program when a new model has to be implemented or when modifications to the models have to be made. To overcome these drawbacks, a more general and user-friendly approach is presented. It consists of a behavioral macromodel that can be used in conjunction with the most commercial SPICE versions. The behavior of the proposed macromodel has been validated by comparing the results with those obtained by BIOSPICE physico-chemical models and experimental measurements. The proposed macromodel is shown to operate also under subthreshold conditions that can be considered as a promising operating mode for large multisensor ISFET-based integrated systems.