Compact model and design considerations of an ion-sensitive floating gate FET

A compact model of an ion-sensitive floating gate FET (ISFGFET) is presented. Unlike in the conventional ISFET, the ISFGFET has a dual gate structure, a sensing gate and a control gate. With the interplay between the control gate and the reference electrode the charging of the fluidic part of the sensor can be controlled and the sensing surface can be programmed. We present an intuitive macromodel with SPICE implementation and discuss the fluidic gating, sensitivity, chemical and electrical tuning of the ionic screening layer as well as the methods to combat the limitations such as the dielectric breakdown of the sensing oxide as well as the design trade-offs of the sensor. Additionally the model allows the integration of this new type of chemical sensor to the CMOS design flow. The proposed model relies on experimentally verified site-binding theory and double layer capacitance formulation explaining the interfacial potential. Unlike previous models we created a unified model where the sensing interface and the FET are coupled and the system is solved as such without unnecessary simplifications used earlier. The model validity is verified by comparing the results with well-established ISFET models and with experimental results.