The fabrication and characterisation of ion-sensitive field-effect transistors with a silicon dioxide gate

Abstract

The ion-sensitive field-effect transistor (ISFET) is a new type of miniaturised semiconductor chemical sensor. It is based on the structure of the metal-insulator-semi-conductor field-effect transistor (MISFET) in which a metal gate is replaced by an ion selective membrane, an electrolyte and a reference electrode. In this paper, an ISFET is fabricated with silicon dioxide as a gate for a pH selective membrane. The measurement of pH sensitivity, selectivity and long-term characteristics of this device have been made. A pH sensitivity can be reached of 40 mV per pH. The oxide gate ISFET device has a significant response to K+ and Na+ ions even at the low concentration of 0.01 mol 1-1. The response to K+ ion is much higher than Na+ ion. The long-term stability of the silicon dioxide gate ISFET is also studied. The long-term drift is about 1 mV per hour after a ten hour immersion.