BioFETs (biologically sensitive FETs) are field-effect biosensors whose transducers consist of a semiconducting material. Their device structure is similar to that of a MOSFET, except that the gate structure is replaced by an aqueous solution that contains the analyte. As the analyte specifically binds to a biofunctionalized surface layer, the charge distribution and potential at the surface change, which in turn changes the conductance of the transducer. This minute conductance modulation thus enables label-free detection within minutes. Therefore this technology enables a vast array of applications and has the potential to revolutionize the biomedical field.
In recent years, experimental sensors based on conventional silicon structures and silicon nanowires have been published. Sensors for the detection of single-stranded DNA and for the detection of tumor markers have been built. Despite these recent successes, a theory to explain the functioning of the sensors and to understand the experiments in a quantitative manner is still lacking. This is mainly due to the complexity of the system, which requires a consistent analysis of the semiconductor, the surface chemistry, the biomolecules, and the transport mechanisms in the aqueous solution.
Therefore we have been developing a new model for BioFETs. Since the length scales of the biomolecules and the sensor area differ by at least 4 or 5 orders of magnitude, we developed a multi-scale model to link the interactions of the electric double layer and the biomolecules with the device simulation at large. The electric double layer at the silicon oxide surface is important for quantitative simulations since it is very effective at screening the charges of and around the analyte from the transducer. Our model also provides self-consistent simulations which are indispensable because of the interactions of the charge carriers in the semiconductor and the ions and biomolecules in the solution.