CMOS Open-Gate Ion-Sensitive Field-Effect Transistors for Ultrasensitive Dopamine Detection

Open-gate ion-sensitive field-effect transistors (ISFETs) are presented in this paper to provide a real-time ultrasensitive dopamine (DA) detection in the femtomolar (fM) range. The polysilicon gates of p-type FETs fabricated in a 0.35-μm complementary metal-oxide-semiconductor (CMOS) process were removed by a convenient post-CMOS process to expose the gate oxide for surface functionalization and biomolecule immobilization. The measured current value increased due to the produced negative charges from binding of the 4-carboxyphenylboronic acid and DA molecules. The thin gate oxide, as the sensing interface, significantly enhances the detection limit, which is comparable to or better than most nanowire-based ISFETs. A self-oscillating readout circuit was used to convert the ISFET current to a digital output for the measurement of multiple sensors, showing the strength of the CMOS-based approach for sensor integration.