Accurate Continuous Monitoring of pH and Blood Gases Using FET-based Sensors Operating in theAmperometric mode

The capacity of ion-selective and the Gas-sensitivefield effect transistors (FETs) to serve as low-power sensors,operating in the amperometric mode for accurate continuousmonitoring of pH and blood gases, is evaluated. A stand-alonecurrent-mode topology is employed in which a constant bias isapplied to the gate with the drain current serving as themeasuring signal. In the amperometric mode, the staticsensitivity of a FET-based sensor given by the product of the FETtransconductance and ? ???⁄?[?] , the sensitivity of thethreshold voltage of the device, ??? to the concentration of themeasurand, ? can be increased by adjusting the device designparameters. Compared with voltage-mode operation (e.g. in thefeedback mode in ISFETs), current-mode topologies offer theadvantages of small size and low power consumption. However,the ion-selective FET (ISFET) and the Gas-sensitive FET(GasFET) exhibit a similar drift behavior imposing a seriouslimitation on the accuracy of these sensors for continuousmonitoring applications irrespective of the mode of operation.Given the slow temporal variation associated with the observeddrift behavior of these devices a post-processing technique, whichinvolves monitoring the variation of the drain current over shortintervals of time, allows extraction of the measuring signal inpresence of drift. Rigorous analytical validation of the method ispresented for GasFET operation in the current mode. Moreover,the correction algorithm is verified experimentally using a Si3N4-gate ISFET operating in the amperometric mode to monitor pHvariations ranging from 3.5 to 10.

Keywords:

Amperometric , Continuous monitoring , Current mode , Drift , GasFET , Gas Sensor , ISFET , Instability , pH Sensor