ISFETs: theory, modeling and chip for characterization

The ISFET (Ion Sensitive Field Effect Transistor) is a structure based on the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) which is capable of measuring ionic concentration of a solution. The ISFET has been used for such areas as DNA sequencing, viruses and bacteria detection. The basic idea behind the ISFETs emerged in 1970, but a deeper understanding of some of its non-idealities and the development of architectures to reduce their effects are still needed. For that reason, this work revisits the basic principles of ISFET operation. The ISFET modeling using the binding site theory, Gouy-Chapman-Stern model and the Advanced Compact Model of the transistor is introduced and implemented in Matlab®. Furthermore, the details of a chip designed on the Virtuoso® platform, aimed at characterizing the ISFETs on the SilTerra D18V technology, are presented. Simulation results estimate an average sensitivity of 45.3 mV/pH for the designed devices over a pH range from 1 to 10. The chip sent for fabrication was kindly supported by Chipus Microeletrônica S.A. and SilTerra Malaysia Sdn Bhd.