The surface potential at the oxide solution interface (Ψ0) is dependent on the pH of the bulk solution (pHB) with the following function: d Ψ0/dpHB=-2.3X(kT)α/q

This is Nernstian in nature but includes a sensitivity parameter α which can vary between 0 and 1. The closer to 1 it is then the more Nernstian the pH response, i.e. it’s closer to acting like a glass membrane ISE, where ΔΨ0 = -59.2 mV/pH at 298K. Theoretically the sensitivity α will have the following formula:

α =1/(2.3kTCCdl /q2βint)+1

This is a function of physical constants (k, T & q) as well as the variables Cdl and βint which represent firstly the double layer capacitance at the solution/oxide interface and secondly the buffer capacity of the oxide surface .

The intrinsic buffer capacity of the oxide surface (βint) is a measure of the ability of the surface to accept or donate protons to and from the solution. It should be maximised to increase the sensitivity by making α closer to unity. Silicon dioxide isn’t the best material to use for an ISFET as the buffer capacity is relatively low. This means the sensitivity is sub- Nernstian and the ISFET made with SiO2 is more sensitive to changes in Cdl. Other materials like silicon nitride (Si3N4), aluminium oxide (Al2O3) and tantalum pentoxide (Ta2O5) are better for a variety of reasons and have higher βint and α. One reason for the better performance of the aluminium and tantalum oxides might be the greater number of oxygen atoms involved in the oxides, meaning a greater density of amphoteric sites at the surface.

Hope, this answer will serve you the basic purpose.