Overcoming the Challenge of Debye Screening in Field-Effect Biosensors

Background: Biosensors based on field-effect devices (bioFETs) offer numerous advantages over current technologies and therefore have attracted immense research over the decades. However, short Debye screening length in highly-ionic physiological solutions remains the main obstacle for bioFET realization. This challenge becomes considerably more acute at the electrolyte-oxide-interface of the sensing area due to high ion concentration induced by the charged amphoteric sites which prohibits any attempt to employ field-effect mechanism to ‘sense’ any charged biomolecules. Results: The presented study was recently accepted for publication with the journal ACS Sensors. We present an electrostatic approach by which the double layer (DL) excess ion concentration is removed, thus forcing the DL ion concentration to match the bulk concentration which subsequently forces bulk screening length at the DL, thereby ‘exposing’ target biomolecules to the underlying bioFET. To this end, we employ local tunable surface electric fields, introduced to the DL using surface passivated-metal electrodes. We examine numerically and analytically the effect of these electric fields on the DL ion distribution. We also numerically demonstrate the feasibility of the proposed approach for a fully-depleted silicon-on-insulator based bioFET and show how the threshold voltage shift induced by the presence of target molecules increases by almost two orders of magnitude upon the removal of the surface excess ion population. Methods: We use Advanced TCAD (Synopsys Inc.) for the numerical calculations, and analytically solve the Poisson equation in the silicon and the Poisson-Boltzmann equation in the electrolyte. Conclusion: We suggest a new paradigm by which the challenge of Debye screening length in bioFETs can be overcome using a simple electrostatic approach.