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. This method, 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 Si
₃
N
₄
-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 Al
₂
O
₃
-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 (g
_m
) suggesting that the optimization of g
_m
can improve the resolution of pH readings. SPICE simulations based on the measured drift data for the Si
₃
N
₄
-gate ISFET indicated an approximately 0.1pH unit improvement in the accuracy of the ISFET for a two-fold increase in the aspect ratio of the channel (W/L).