Recent advances in microelectronic ion-sensitive devices (ISFETs). The operational transducer

Abstract

The development of ion-selective field-effect transistors (ISFETs), their advantages over ion-selective electrodes (ISEs), and their intrinsic disadvantages are reviewed. Some of these disadvantages can be overcome by utilising the ISFET as the integral element in an analogue circuit to provide an operational transducer on a 2 × 2.5 mm silicon chip. The resultant device has small thermal sensitivity and linear response. The performance of a nitrate ion-sensitive device, appraised by a continuous dilution, computer-controlled technique, is described. Successful, and adequate, encapsulation of the semiconductor excluding the chemosensitive gate region is mandatory, and recent improvements to the polyimide–photopolymer process are described.