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The integration of various types of semiconductor devices with particular electrochemically active thin films makes possible a new generation of integrated-circuit chemical microtransducers which have many important (and unique) advantages over their conventional antecedents, including small size, robust solid-state nature and the potential for low-cost mass-fabrication; attributes which are particularly appropriate for biomedical usage. The prime semiconductor device for such applications is the MIS transistor. first modified into an ion-sensitive (chemical-sensitive) field-effect transistor (ISFET, ChemFET) in 1970. The paper reviews the development of ChemFET devices since their inception, and describes the important aspects of their theory, structure and design with particular emphasis on the practical features of device operation. As an illustrative example, a novel triple-function chemical-sensing IC for the simultaneous measurement of K+, H+ and Ca2+ in aqueous solutions is described.