To maintain device stability and minimize electrical noise, a commonly used reference electrode such as Ag/AgCl, Ag wire, or Pt wire is used as a solution gate controller and is kept at ground potential. CNTs act as a sensing element in the FET device, which differs from conventional planar FETs, and their enhanced sensitivity is attributed to their reduced dimensionality and large surface-to-volume ratio, making them highly responsive to any electric bias imposed on the nanowire interface. In p-type nanobiosensors, the receptors on the nanowire interface capture positively charged analytes, resulting in an excess of holes that dominate the CNTs-FET. This causes a depletion of charge carriers and a reduction in the source-drain conductance or current in the channel. This principle can also be extended to examine n-type CNTs-FET devices. Currently, CNTs configured as FETs are utilized to convert bio/chemical charge variations into electrical signals for in vitro and in vivo applications based on target-receptor binding. These include pH, nucleic acids, and proteins that are useful biomarkers for AD diagnosis.