CMOS Readout Circuit Developments for Ion Sensitive Field Effect Transistor Based Sensor Applications

Abstract

Biomimetic devices have become more and more important in modern life where populations are aging; and the applications of electronic tongue system to water quality and environmental monitoring have become a significant field all over the world. Electronic tongue system uses sensor arrays and signal processing techniques such as identification, classification and recognition for quantitative multi-component analysis and for artificial assessment of taste and flavor of various liquids (Cjosek & Wroblewsk, 2007). Ion Sensitive Field Effect Transistor (ISFET), an electrochemical and potential type sensor, has served as excellent candidate for various electronic tongue applications. The ISFET, invented by Bergveld in 1970, is a solid-state device that combines a chemically sensitive membrane with a MOS type field-effect transistor (Bergveld, 1970). Due to its small size, rapid pH response and rugged solid-state construction, the ISFET exhibits a number of advantages over conventional pH-glass electrodes. ISFET has been extensively studied in past 36 years (Bergveld, 1991 and 2003; Garde et al., 1995). The current status and trends of main ISFET-based research, shown in Fig.1, are (1) single and sensor array applications, (2) ISFET micro-system fabrication in a standard CMOS technology, and (3) diversified ISFET-based biosensor development. For example, the ISFET research topics in Taiwan for the past ten years (Yin et al., 2001; Chin et al., 2001; Chung et al., 2004, and 2008) are focused on the study of new sensing material, on fabrication technology and device structure development, on diversified field applications, on the study and improvement for non-ideal characteristics, and on new readout circuit development. Based on our previous researches, the key problems in readout circuit development are due to the inherent characteristics of ISFET and to the body effect caused by common substrate of sensor array applications. The inherent characteristics of ISFET, like time drift and temperature dependency, cause