Multi-stack insulator to minimise threshold voltage drift in ZnO FET sensors operating in ionic solutions

Abstract

FET biosensors operating in an electrolyte experience a monotonic, temporal and relatively slow change in threshold voltage caused by the hydration of the insulator layer between the electrolyte and the FET's channel. Minimising this temporal change in threshold voltage is critical as, over time, the drain current of n-channel FETs decreases, making it difficult to distinguish between the signal generated in response to analyte - receptor binding events and the background noise generated by the electrolyte and the FET biosensor. While Rapid Thermal Annealing of the insulator layer is known to diminish threshold voltage drift and its negative effects, it is not compatible with a low temperature fabrication process of 200 °C. Our low temperature approach to minimising threshold voltage drift involves depositing a tri-layer insulator stack, consisting of a layer of HfO2 between two Al2O3 layers. Wetting ZnO NWFETs with PBS (10 mM phosphate, 150 mM KCl, pH 7.4) for an hour, showed that ZnO NWFETs with a stack insulator layer experienced a much smaller threshold voltage and drain current drift (100 mV, 0.064 nA) than ZnO NWFETs with a single material insulator layer (≥4300mV, 2.72 nA), Aluminium oxide in this case. Having established the resilience enhancing properties of the stack insulator layer on FETs operating in electrolytes of physiological relevant ionic concentrations; ZnO NWFETs with a stack insulator layer were shown to be capable of detecting the presence of the miDNA-21 strands. This, in effect, paves the way for miRNA sensing experiments in the near future and for exploring the potential of ZnO NWFETs as a diagnostic tool.

Keywords

• Field effect transistor biosensors
• Zinc oxide nanowire field effect transistor
• Stack insulator layer
• Threshold voltage drift
• Phosphate buffer solutions
• microDNA