Kinetics of the hydration reaction at the electrolyte–insulator interface

Abstract

The instability of the dc operating point in the pH-sensitive ion-selective field effect transistors (ISFETs) has been ascribed to a chemical ageing at the electrolyte–insulator. This instability, commonly referred to as a drift, is believed to involve formation of a chemically-modified insulator surface layer as a result of hydration of the insulator material. A kinetic model for hydration of the amorphous insulator material is presented. The kinetics of hydration is limited by the hopping and/or trap-limited transport mechanism known as dispersive transport, the key characteristic of which is a power-law time dependence of the diffusion coefficient. The power-law time dependence of the diffusion coefficient will be shown to lead to a stretchedexponential decay in the form exp[–(t/τ)β] for the excess density of sites or traps occupied by the hydrating chemical species undergoing dispersive diffusion, where τ is the time constant associated with a structural relaxation and β is the dispersion parameter satisfying 0 < β < 1. The kinetics associated with a hydration reaction limited by the dispersive diffusion has been shown to lead to a hydrated layer thickness exhibiting a time dependence in the form {1–exp[–(t/τ)β]}. The first order rate equation describing the kinetics of the hydration reaction is characterized by the time-dependent rate coefficient.