Current-Mode Signal Enhancement in the Ion-Selective Field Effect Transistor (ISFET) in the Presence of Drift and Hysteresis

The accuracy of the ion-selctive field effect transistor ( ISFET) is limited by errors ascribed to drift and hysteresis. In this work operation of the ISFET as a stand-alone pH sensor operating in the current mode is demonstrated, the dependence of drift on pH is characterized and modeled, and the relationship between drift and hysteresis is investigated. Also, a post-processing method for extraction of the ISFET current signal in the presence of drift and hysteresis is formally developed. Thismethod, which is based on sampling the drain current over relatively short time intervals, is verified experimentally in the presence of drift and hysteresis by monitoring step changes in pH using a Si3N4-gate ISFET biased in the triode region with the pH of the solution cycled up and down over a seven-unit range. The corrective algorithm was also demonstrated by extracting the measuring signal using an Al2O3-gate ISFET operating in the current mode to monitor pH variations ranging from 4 to 10. The theoretical basis of the proposed method is validated by developing the concept of signal-to-drift ratio as a figure of merit for the resolution of pH-sensitive ISFETs. The signal-to-drift ratio is shown to increase in proportion to the ISFET transconductance ( gm) suggesting that the optimization of gm can improve the resolution of pH readings. SPICE simulations based on themeasured drift data for the Si3N4 -gate ISFET indicated an approximately0.1pH unit improvement in the accuracy of the ISFET for a two-fold increase in the aspect ratio of the channel (W/L).

Automated summary

This study investigates the drift and hysteresis issues in ISFET as a stand-alone pH sensor, proposes a sampling-based post-processing method, and validates it experimentally. The corrective algorithm improves the accuracy and resolution of ISFET by addressing the drift and hysteresis problems.