IMPLANTABLE ION-SENSITIVE TRANSISTORS.

Abstract

The optimization of ion-sensor transducers for long-term implantable pH sensing applications has been achieved by increasing the stability and linearity of the ion-sensitive field effect transistor (ISFET) through the use of multiple site materials, providing an on-chip Ag/AgCl reference electrode, and improving encapsulation reliability by junction-isolating the ISFET device. The devices also features polysilicon interconnects to the source and drain and borosilicate glass as the pH sensitive gate dielectric material. Borosilicate glass has several advantages over silicon nitride and silicon dioxide ISFETs. It is a stable dielectric in the presence of aqueous solutions, the response does not degrade with time, and it can be easily made from materials common to IC fabrication technology. Two site models are developed to model the borosilicate glass and silicon nitride devices. Using materials with more than one type of site is shown to be a reliable method for achieving a linear nernstian response.