Noise considerations in field-effect biosensors

Field-effect sensors used to detect and identify biological species have been proposed as alternatives to other methods such as fluorescence deoxyribonucleic acid (DNA) microarrays. Sensors fabricated using commercial complementary metal-oxide-semiconductor technology would enable low-cost and highly integrated biological detection systems. In this paper, the small-signal and noise modeling of biosensors implemented with electrolyte-insulator- semiconductor structures is studied, with emphasis on design guidelines for low-noise performance. In doing so, a modified form of the general charge sheet metal-oxide-semiconductor field-effect transistor model that better fits the electrolyte-insulator-semiconductor structure is used. It is discussed how if the reference electrode and the insulator-electrolyte generate no noise associated with charge transport, then the main noise mechanisms are the resistive losses of the electrolyte and the low-frequency noise of the field-effect transistor. It is also found that for realistic sensor geometries and high electrolyte concentrations, the noise from the field-effect transistor (FET) dominates the thermal noise from the electrolyte resistance, and the optimal biasing point for the FET for minimum noise is found to be around moderate inversion.