H $${^{+}}$$ + -ion-sensitive FET macromodel in LTSPICE IV

Abstract

pH measurements are widely used in biomedical engineering, agriculture, environmental studies, the food industry, etc. In biomedical and healthcare research, it is said that all aqueous samples have their pH tested at some point in their lifecycle for evaluation of wound healing or susceptibility, diagnosis of diseases, cellular internalization, etc. The ion-sensitive field-effect transistor (ISFET) in blank form is capable of pH measurements. When its gate comes into contact with an electrolyte, the activity of ions dissolved in the electrolyte is converted to an electrical potential while releasing free electrons. Such use of the ISFET has become popular, as it allows sensing, preprocessing, and computational circuitry to be encapsulated on a single chip, enabling miniaturization and portability. LTSPICE IV is a computational electronic design software package that facilitates design and simulation of circuits before their costly fabrication. Despite its popularity, the ISFET is still not available in the component library of LTSPICE IV. Hence, we propose herein an ISFET macromodel to contribute as a new subcircuit block for the LTSPICE IV component library, to enable design of ISFET-based sensors or microsystems. A mathematical model for the ISFET is first derived and programmed into netlists, from which a macromodel is constructed. This ISFET macromodel is tested by using it to generate characteristic graphs relating important ISFET parameterization variables. Simulation results show that, at low pH, when the H$${^{+}}$$+ ion concentration in the electrolyte is high, higher drain current is generated, similar to the results of the mathematical model. As a consequence, from the simulation of the drain current with respect to a reference voltage, at higher pH values the first-order response is found to be more sluggish with slow turn-on of the drain current. Moreover, the drain voltage is found to suppress the sensitivity (reference threshold voltage/pH) of the ISFET. According to the simulations of the drain current and pH, reference voltages of 1.0 and 1.5 V result in high linearity and sensitivity. According to the simulations of the drain current and drain voltage, a reference voltage of 1.5 V is found to be good in terms of operational linearity and sensitivity.