6.1 Sensing via the Long-Range Field-Effect

Since most of the biological polymers are inherently charged, the key of the working principle is to exploit the intrinsic charges in the macromolecules. Therefore, if one is able to bring the macromolecule close enough to the surface of the dielectric, the intrinsic charges of the macromolecules will cause a formation of counter charges in the semiconductor by the field-effect. Due to the counter screening of the salt ions in the solute and the relatively wide distance (at least the OHP) compared to the hydrogen charges directly at the oxide interface, the sensitivity is expected to be lower than for an ISFET exploiting pH changes. Many effects, involving the ionic counter charge around the molecule and the surface charge of the dielectric, are able to diminish the field-effect of the macromolecule in the semiconductor. Therefore, it was believed that the detection of macromolecules like DNA is not feasible [196]. However, many experiments disproofed this assumption and showed the practicability of such an approach [17,152,213]. Fig. 6.1 illustrates the long-range field-effect for large macromolecules e.g. proteins or DNA.

This device type, termed as BioFET, exhibits several advantages compared to currently established methods. The biggest advantage against common solutions is the label-free operation of BioFETs. Instead of labeling the analyte with fluorescent or radioactive probes and a subsequent readout step by an appropriate detection technique, the BioFET device allows a simplified analyte preparation and a direct readout via an electrical signal, thus, saving time, expenses and laboratory equipment. Furthermore, the label-free technique, enables near real-time sensing and a high sensitivity. Exploiting the specifity of a given chemical reaction (``key-lock'' principle), the sensor can be adapted for a wide range of molecule classes via exchanging the functionalization of the dielectric surface.