Quasi-synchronous thermocompensation for ISFET-based ionometric devices. Part 1: Theory and simulation

Solid-state ion selective transducers, as an alternative to the traditional liquid electrolyte-filled glass electrodes, are known for over four decades now, and find their use in various areas of industry and applied science, such as in vivo analysis of the ions activity in biological and medical research, monitoring of toxic and aggressive environments, and biosensors design. However, along with potential advantages — short response time, small size, chemical inertness and durability — solid-state devices also possess certain inherent drawbacks — namely intrinsic noise, drift and instability of sensing properties, and cross-sensitivity to various interfering environmental conditions — that inhibit their widespread acceptance. Further improvement of the fabrication technology and methodology of application of these devices is thus still an important practical task even today. This paper is a first part of the two-part work dedicated to the problem of compensating the temperature dependence of a solid-state ion selective transducer output. Specifically, presented work considers the possibility of using ion-selective field-effect transistors (ISFET) that serve as primary transducers in an ionometric device, as temperature sensors. This allows compensating the temperature dependence of ionometric signal without substantial complication of the ionometer structure, and eliminates the need to include a separate thermometric channel as part of the instrument. Ionometric and thermometric channels are combined into a unified measuring path, with the sensor functions separated in time. The ISFET operation modes are switched by changing polarity of the bias voltage, and thus direction of the current flowing through the sensor. The authors propose a corresponding secondary transducer structure and simplified schematic illustrating the implementation of its key components. The concept’s applicability is supported by the circuit simulation results. Some aspects of the practical implementation of the proposed concept will be presented further in the upcoming second part of the paper.