2.5 Working Principle of a BioFET

Figure 2.9: The working principal of a BioFET. If charged, sample molecules attach to the receptors at the biofunctionalized surface and the potential within the semiconductor changes. This causes a change in the resistance of the field-effect transistors channel.

A BioFET contains the following parts: a semiconductor transducer, a dielectric layer, a biofunctionalized surface, the analyte, and a reference electrode (the gate in FET terms) as shown in Fig. 2.9. The semiconductor transducer is realized by a conventional field-effect transistor. The dielectric layer is an oxide (e.g. $SiO_{2}$) and has two tasks the first is to isolate the channel of the FET from the liquid and the second is to electrostatically couple the surface layer charge into the channel. On top of the dielectric is a biofunctionalized layer which exhibits immobilized biomolecule receptors able to bind the desired molecule. The analyte is a solution which contains the dissolved sample molecules. The reference electrode allows to adjust the device so its sensitivity will be maximized (the optimum lies around moderate inversion [160]). If the target molecules bind to the receptors, a change in the surface charge density occurs. This change alters the potential in the semiconductor and thus the conductivity in the channel of the field-effect transducer. The chemical reaction of the sample and receptor molecules takes place at the Angstrom length scale, while the BioFET is in the micrometer length scale. This points out the importance of a proper mathematical description of the solution/semiconductor interface.