The operation principle of a CNT-FET biosensor is based on the changes in electrical conductance of the carbon nanotube channel caused by the presence of biomolecules (Figure ). When biomolecules bind to the surface of the carbon nanotube, they alter its charge density, which in turn changes the electrical conductance of the nanotube. The changes in electrical conductance can be measured by a gate electrode in close proximity to the carbon nanotube, allowing for the detection and quantification of the biomolecules. A CNTs-based FET device commonly employs a third gate electrode that is capacitively coupled to apply an electric field and modulate the conductivity of the semiconductor channel, which is connected to metal source and drain terminals.