Ion-selective field-effect transitors (ISFETs)

Abstract

Most microelectronic chemical sensors are based on the insulated-gate field-effect transistor (IGFET), where the insulator-semiconductor is silicon dioxide-silicon, overlain by the ion-blocker, silicon nitride, and an additional layer conferring alternative ion-sensitivity and selectivity. Materials used to confer ion-sensitivity include, besides silicon nitride (H+), aluminium and tantalum oxides (H+), special glasses (H+, Na+, K+), valinomvcin (K+), tetraalkylammonium salts (Cl- , NOj), and various synthetic ionophores (Ca 2 +, Na+). The metal gate connection of the FET structure is replaced by a reference electrode in the solution containing the ion to be determined (analyte). The problems of design, construction and use of ion-selective FETS (ISFETS) as biosensors are surveyed and illustrated by work in Newcastle on ex vivo monitoring of blood during surgery for potassium and calcium ions and pH.