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

The capacity of ion-selective and the Gas-sensitive field effect transistors (FETs) to serve as low-power sensors, operating in the amperometric mode for accurate continuous monitoring of pH and blood gases, is evaluated. A stand-alone current-mode topology is employed in which a constant bias is applied to the gate with the drain current serving as the measuring signal. In the amperometric mode, the static sensitivity of a FET-based sensor given by the product of the FET transconductance and ∂ V_TH⁄(∂[M]), the sensitivity of the threshold voltage of the device, V_TH to the concentration of the measurand, M can be increased by adjusting the device design parameters. Compared with voltage-mode operation (e.g. in the feedback mode in ISFETs), current-mode topologies offer the advantages 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 serious limitation on the accuracy of these sensors for continuous monitoring applications irrespective of the mode of operation. Given the slow temporal variation associated with the observed drift behavior of these devices a post-processing technique, which involves monitoring the variation of the drain current over short intervals of time, allows extraction of the measuring signal in presence of drift. Rigorous analytical validation of the method is presented 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 pH variations ranging from 3.5 to 10.