Capacitive control of an ISFET using dielectric coated electrodes

Reliable control of ISFETs encapsulated in microfluidic environments is required to achieve portable and reliable biosensors. Conventionally ISFET´s maintain the stable electrochemical potential through a reference electrode, which can be consumed and is difficult to miniaturize. In this paper, to facilitate microfluidic packaging by using an on-chip capacitive reference electrode, we employ a modified CνMOS approach by connecting the sensing gate (SG) of the CνMOS to an uncoated platinum electrode, while another large-area gate (Si₃N₄ covered platinum) in the fluidic chamber serves as the electrolytic control gate (ECG). The capacitance ratio between the ECG to the extended SG needs to be engineered to efficiently modulate the transconductance. We observe an I_on/I_off ratio of 10⁹ and a subthreshold slope of ~80mV/decade when the ISFET is controlled using the ECG. We further observe that the extended platinum SG is relatively insensitive to saline concentration which we attribute to the lack of ion specificity at the extended platinum electrode. The pH response, on the other hand, is anomalous. We attribute this to the opposing proton-dependent sensitivities at the exposed Pt and Si₃N₄ coating of the ECG. Impedance spectroscopy however shows a clear molarity dependent RC time constant shift indicative of clear conductivity dependence. Our results suggest that non-faradaic capacitive control of ISFET´s are more suitable for applications involving impedance detection and not ionic sensing.