Electrochemical gating on CMOS: Interplay of field, acidity and salinity on an electrolyte-insulator interface

When an insulator-covered electrode is biased in an electrolyte, the emanating field imparts control over the electrical double layer at the interface. This field not only influences the electrochemical potential of ions within a Debye length, but also perturbs the insulator surface charge, the proton binding affinity, adsorption equilibrium and the net charge within the double layer. The interplay between the applied electric field and the chemical equilibrium at the interface is termed as electrochemical gating [1]. Ion sensitive transistors (ISFET) [2] use a similar principle to detect charges in solution via the exposed gate dielectric. The combined effects from acidity, field and salinity at such interfaces have not been well characterized against physical models. We find that proton binding to surface groups is affected by the oxide field and salinity, which together sets the surface potential ψ₀. We present a 2-pK surface reaction model that provides a consistent explanation on the combined effects. We further demonstrate electrochemical gating integrated on a commercial CMOS platform with floating gate transistors. This provides a simultaneous capability of sensing and actuation.