Methods of isfet fabrication

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The Ion Sensltlve Field-Effect Transistor (ISFET) 1s a new, integrated device composed of a conventional ion selective electrode and an InsulatedGate FET(IGFET) The device 1s slmllar to the conventional IGFET except that the metal gate electrode 1s removed m order to expose the underlying msulator layer to solution The gate msulator plays the role of an ion selective electrode Figure 1 shows the prmclple of the ISFET The first ISFET was reported by Bergveld [ 11, and V~IXOUS devices of this type have been described m recent papers [ 2 61 ISFETs have potential advantages over conventronal zon selective electrodes m theurapid response, small size, and low output impedance Addltlonally , the use of IC technology allows the fabrlcatlon of ISFETs for the detection of several ion species sunultaneously These advantages of ISFETs are extremely attractive for P1omedlca.l apphcations The nature of ISFETs requires that they are used m solutions of electrolytes which, from the electronic point of mew, are hostile enwonments The electrical mtegnty of the whole ISFET assembly 1s cntlcal and a prerequlslte for the stable operation of those devices, and, especially, the protection of the gate msulator from ion migration and hydration 1s crltlcally unportant For these reasons, the charactenstlcs of ISFETs are greatly dependent on the structure and the method of fabrication of the device

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