Abstract
For the miniaturization of ISFET sensing systems, the concept of a REFET with low ion sensitivity is proposed to replace the conventional reference electrodes through the arrangement of a quasi reference electrode and a differential readout circuit. In this study, an ion-unblocking membrane was used as the top layer of a REFET. To optimize the REFET performance, the influences of the silylating process, different plasticizers, and the composition of the PVC cocktails were investigated. A low sensitivity (10.4 ± 2.2 mV/pH) and high linearity (99.7 ± 0.3 %) in the range from pH 2.2 to pH 11.6 was obtained for the REFET with a 60 wt.% DNP/(DNP + PVC) membrane. To evaluate the long term stability, the drift coefficient was estimated, and for the best REFET, it was −0.74 mV/h. Two criteria for assessing the lifetime of REFETs were used, namely the increase in pH sensitivity to a value higher than 15 mV/pH and the degradation of linearity below 99 %. For the best REFET, it was approximately 15 days.
Highlights
The ion sensitive field effect transistor (ISFET) was first proposed by P
Because the device structure and fabrication process are similar for metal oxide field effect transistors (MOSFETs) and ISFETs, both devices can be manufactured by CMOS technology and miniaturized to the micrometer scale [2]
In order to decrease the pH sensitivity of the ISFET for REFET application, the polyvinyl chloride (PVC) membranes were deposited on the open gate windows of ISFETs
Summary
The ion sensitive field effect transistor (ISFET) was first proposed by P. From the state-of-the-art analysis results, the short lifetime of miniaturized REs with small internal electrolyte volume must still be improved [11,12] To solve this problem, the concept of a differential system with an ISFET/REFET (reference field effect transistor) pair was first introduced by Matsuo in 1978 [13]. The concept of an ISFET/REFET differential pair is applicable to pH sensing applications, and to monitoring concentrations of other ions, such as Na+ and K+ [14], as well as other species, such as creatinine and urea, with the use of chemically and enzymatically modified field effect transistors (ChemFET, EnFET, respectively). To evaluate the sensing properties of REFETs, the sensitivity to hydrogen ions, transconductance compatibility, drift coefficient, and lifetime were studied
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