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This paper describes the development of ISFETs in an historical setting, but is not limited to that. Based on the development regarding the theory, the technology, the instrumentation and the experience with many specific applications, also future projects are defined, such as concerning cell acidification, REFET biasing and a complete new range of FET sensors based on local pressure induction by (bio)chemical interaction with immobilised charged molecules (hydrogels). Also the present patent and market position is discussed. It is concluded that in the past 30 years the ISFET research and development made continuous progress on a regular base, but the practical applications stayed behind, especially concerning the dynamic use of ISFETs in combination with an integrated pH actuator. The newly proposed research projects may be good for an other 30 years.

Polydimethylsiloxane (PDMS) is a commercially available physically and chemically stable silicone rubber. It has a unique flexibility with a shear elastic modulus G ≈ 250 kPa due to one of the lowest glass transition temperatures of any polymer (T g ≈ −125 • C). Further properties of PDMS are a low change in the shear elastic modulus versus temperature (1.1 kPa • C −1), virtually no change in G versus frequency and a high compressibility. Because of its clean room processability, its low curing temperature, its high flexibility, the possibility to change its functional groups and the very low drift of its properties with time and temperature, PDMS is very well suited for micromachined mechanical and chemical sensors, such as accelerometers (as the spring material) and ISFETs (as the ion selective membrane). It can also be used as an adhesive in wafer bonding, as a cover material in tactile sensors and as the mechanical decoupling zone in sensor packagings.

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Colloid chemists have proposed several theories to describe the charging mechanism of metal oxides in electrolyte solutions and the resulting electrical double layer at the oxide surface. In this paper a new general theory to describe the electrostatic potential at the metal oxide electrolyte solution interface is presented. This theory describes the variations of the electrostatic potential as a function of the differential double layer capacitance and the intrinsic buffer capacity. ISFET measurements are interpreted using this theory, and it is shown that these measurements can differentiate between the theories for the double layer and the theories for the charging mechanism for the oxide.