CHAPTER 4 I ON -S ENSITIVE F IELD -E FFECT T RANSISTOR (ISFET)­B ASED C HEMICAL S ENSORS V. K. Khanna 1. INTRODUCTION Chemical sensors include sensors for gases and chemical compounds in aqueous solutions, either biomolecules or ionic species. A chemical sensor is a miniature device comprising a recognition element, a transduction element, and a signal processor capable of continuously and reversibly reporting a chemi- cal concentration. For sensing, various approaches--electrochemical, optical, etc.--may be used. The field of electrochemical sensors based on a microelectronic device, the ion-sensitive field-effect transistor (ISFET), has experienced spectacular growth during the period spanning approximately four previous decades, starting from 1970. These sensors find numerous applications, including home safety, food freshness, medical diagnosis, and environmental protection, to name a few, and are therefore likely to have a prolific impact on our lives in the near future. The ISFET, introduced by Piet Bergveld (Bergveld 1970, 2003a, 2003b), is essentially a pH sensor and provides a platform for fabrication of a diversity of chemical sensors (including biosensors) (Yuqing et al. 2003). It may be mentioned here that the ISFET has also been used as a physical sensor for mea- suring flow velocity, flow direction, and diffusion coeffi cient by using in situ electrochemically generated OH - ions through the electrolysis of water solutions (Poghossian et al. 2003). Structurally, the ISFET is a modified metal-oxide semiconductor field-effect transistor (MOSFET) in which the gate metal has been replaced by an aqueous solution and a reference electrode immersed in the solution (Figure 4.1) (Khanna 2007a, 2007b). The metallic contact of the reference electrode is 171