Parameter Selection for an Electrolyte Insulator Interface based Si3N4 Field Effect Transistor Sensitive to H+ Ion Concentration with PSpice Macro Modeling

摘要

Recent advances in the electrochemical sensor and silicon technology has made integration of ISFET sensor with signal processing easier, thus enabling simpler and portable application, even potential on-site screening. ISFET sensor fabricated with CMOS technology benefits from low cost production, low power and miniaturization enabling for microsystem. ISFET sensor is fundamentally a MOSFET with a gate structure comprising of a reference electrode and insulator. The ion concentration of electrolyte which completes the gate-source circuit, effects the gate potential to produce threshold voltage. It serves at the front end of the instrumentation system, with a critical role to interface between the electronic signals and measured signals. OrCAD PSpice facilitates the design and testing of circuitry before the costly fabrication, with a drag-n-drop sub-circuit block library of macro models. However, even with its current popularity, macro model for ISFET devices is not found. The paper proposes a macro modeling approach for the physical-chemical behavioral model of ISFET, to contribute to a new sub-circuit block for PSpice, to allow characterization and parameterization of such devices to be simulated. Its functional quality is ascertained by comparing its drain current characteristic against that generated from source code from previous work, with ±8% discrepancy in sensitivity for pH [4 7 10]. Then, it is used to design parameters for a Si3N4 FET sensitive to H+ ion, for operation characteristic to be as linear and sensitive as possible. The drain voltage and Vbias optimal for this requirement are found to be O.lvolt and 1.5volt respectively. In the case of drain voltage, it is found that smaller voltage produces faster and more sensitive response. Higher drain current and lower cut-off voltage yields higher sensitivity. At the optimal drain voltage of O.lvolt, a sensitivity of 54.79mV/pH is reported. In the case of Vbias, Vbias of 1.5volt is preferred to 1 .Ovolt for linear change in drain current to pH value.