380 A Generalized Theory of an Ag+-sensitive Electrolyte-Insulator-Semiconductor Field-effect Transistor with Silica Surface Modified by Chemical Grafting Sensors and Actuators, BI (1990) 380-384 H. PERROT, N. JAFFREZIC-RENAULT and P. CLECHET Laboratoire de Physiochimie des Interfaces-U.R.A. CNRS 404, Ecole Centrale de Lyon, BP 163, 69131 Ecully Ckkx (France) W. B. WLODARSKI Royal Melbourne Institute of Technology, G.P.O. Box 2476V, Melbourne, 3001 Vie. (Australia) N. F. DE ROOIJ and H. H. VAN DEN VLEKKERT University of NeuchBtel, Institute of Microtechnology, Rue A.L. Breguet, CH-2ooO Neuch&el (Switzerland) zyxwvutsrqponmlkjihgfedcbaZY Abstract The feasibility of an Ag+-sensitive electrolyte- insulator-semiconductor field-effect transistor (EISFET) with silica surface modified by chemical grafting has been previously proved. Fabricated EISFET sensors feature a long lifetime and a fast response. The main purpose of this paper is to provide further insight into the chemical processes that govern the chemical sensitivity of an Ag+-sensi- tive EISFET sensor. A first order theoretical model is developed that allows the potential at the insulator-electrolyte interface, the threshold voltage potential and the gate voltage of an Ag+- sensitive EISFET sensor, with silica surface modified by chemical grafting, to be determined. The site-binding model has been applied to the modified silica/electrolyte interface. The model successfully explains the Ag’ sensitivity as well as the H+ ion interference effect on the EISFET as an Ag+ sensor. Some discussion of the parameters influencing the Ag+ sensitivity has been presented. From this model, it is concluded that the cyanografted site density, NsCN, and the complex- ation constant, pKcN , are the main controlling factors for the EISFET as an Ag+ sensor. For high sensitivity, large NsCN and pKcN values are required. This provides guideline for selecting a proper chemical grafting process to achieve im- proved Ag+ performance. In this study, a one-dimensional model with the insulator surface potential, Y,, assumed to be constant along the direction of the channel is used. The current-voltage (terminal) character- istic of the EISFET is then derived in a manner similar to the derivation of MISFET char- acteristics, in which ‘I’,, is obtained by solving the system of equations for the EIS structure. The overall model is used to predict the manner 09254005/90/$3.50 in which the gate voltage varies with the Ag+ concentration. The presented model can be adapted to other ion-sensitive EISFET sensors. Introduction Among chemically sensitive solid-state devices, the interest for electrolyte-insulator-semiconduc- tor field-effect transistor (EISFET) has increased sharply in recent years. For developing commer- cially viable devices, progress has to be done in fabrication and packaging, in elaborating new ion-sensitive membranes and in understanding the basic mechanisms involved in ionic detection. The feasibility of an Ag+-sensitive electrolyte- insulator-semiconductor field-effect transistor (EISFET) with silica surface modified by chemical grafting has been previously proved. Fabricated EISFET sensors feature a long lifetime and a fast response [ 11. The main purpose of this paper is to provide further insight into the chemical processes that govern the chemical sensitivity of an Ag+-sensi- tive EISFET sensor. A first order theoretical model is developed that first allows the threshold voltage potential of an Ag+-sensitive EISFET sensor to be calculated versus pAg, the site bind- ing model being applied to the modified silica/ electrolyte interface. Then, the standard MISFET device theory can be used to obtain an overall model to predict how the gate voltage Vo varies with pAg when the drain current, the drain voltage and the temperature are maintained constant. Theoretical model and experimental measurements are compared. Some modified insu- lator/electrolyte interface parameters and some solid-state device parameters are calculated through this model. 0 Elsevier Sequoia/Printed in The Netherlands