Abstract
The Field Effect sensors are broadly used for detecting various target analytes in chemical and biological solutions. We report the conditions under which the pH sensitivity of an Ion Sensitive Field Effect transistor (ISFET) sensor can be significantly enhanced. Our theory and simulations show that by using pH buffer solutions containing counter-ions that are beyond a specific size, the sensor shows significantly higher sensitivity which can exceed the Nernst limit. We validate the theory by measuring the pH response of an extended gate ISFET pH sensor. The consistency and reproducibility of the measurement results have been recorded in hysteresis free and stable operations. Different conditions have been tested to confirm the accuracy and validity of our experiment results such as using different solutions, various oxide dielectrics as the sensing layer and off-the-shelf versus IC fabricated transistors as the basis of the ISFET sensor.
Highlights
The Field Effect sensors are broadly used for detecting various target analytes in chemical and biological solutions
The 20 nm Al2O3 layer is placed on top of the extended gate electrode as the sensing dielectric while the pH of point of zero charge is set to pH 8
The sensitivity of Floating Gate (FG) potential to the change in bulk pH solution is considered as the pH sensitivity of the sensor and all the ion sizes are considered to be their hydrated values in the solution
Summary
The Field Effect sensors are broadly used for detecting various target analytes in chemical and biological solutions. Our experimental sensor is an EG-ISFET; the sensing gate is formed by extending out the gate of an nFET transistor and depositing a thin Al2O3 layer on the extended gate as the sensing dielectric. In this structure, the area of Sensing Gate (SG) compared to the area of entire sensor is one of the structural design parameters that greatly influences the electrical behavior of the sensor due to the existence of parasitic capacitance which degrades the measured sensitivity of the sensor (in Supplementary information section I, we elaborate more on this). Differentiating eq 1 with respect to the bulk pH and using eq 4, the sensitivity of surface potential to the bulk pH can be derived as[14]: dψ0 dpHB
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