Electrolyte screening is well known for its detrimental impact on the sensitivity of liquid-gated field-effect transistor (FET) molecular sensors and is mostly described by the linearized Debye−Huckel model. However, charged and pH-sensitive FET sensing surfaces can limit the FET molecular sensitivity beyond the Debye−Huckel screening formalism. Pre-existing surface charges can lead to the breakdown of Debye−Huckel screening and induce enhanced nonlinear Poisson−Boltzmann screening. Moreover, the charging of the pH-sensitive surface groups interferes with biomolecule sensing resulting in a pH interference mechanism. With analytical equations and TCAD simulations, we highlight that the Debye−Huckel approximation can underestimate screening and overestimate FET molecular sensitivity by more than an order of magnitude. Screening strengthens significantly beyond Debye− Huckel in the proximity of even moderately charged surfaces and biomolecule charge densities (≥1 × 10 12 q/cm 2 ). We experimentally show the strong impact of both nonlinear screening and the pH interference effect on charge-based biomolecular sensing using a model system based on the covalent binding of single-stranded DNA on silicon FET sensors. The DNA signal increases from 24 mV at pH 7 to 96 mV at pH 3 in 1.5 mM PBS for a DNA density of 7 × 10 12 DNA/cm 2 . Our model quantitatively explains the signal's pH dependence with roughly equal nonlinear screening and pH interference contributions. This work shows the importance of reducing the net charge and the pH sensitivity of the sensing surface to improve molecular sensing. Therefore, tailoring the gate dielectric and functional layer of FET sensors is a promising route to strong silicon FET molecular sensitivity boosts.

We experimentally investigate the influence of non-linear electrolytic screening by the electric double layer (EDL) and its impact on the field-effect transistor (FET) sensitivity to charged (bio)molecules. We use DNA hybridization to a PNA capture probe layer as model system. By co-depositing positively or negatively charged blocker molecules together with the PNA capture probes on the negatively charged SiO 2 surface of the FET, we control the overall surface charge and modulate the strength of local ion concentration within the EDL. We observe a FET signal enhancement of up to 51% when the overall charge including the captured analyte itself swings roughly symmetrically around zero during capture, as confirmed by zeta potential measurements. Surface plasmon resonance measurements rule out a change in captured analyte density as the origin of the enhancement in sensitivity. This confirms that excess screening caused by the large local ion concentration, which increases non-linearly with potential in the EDL, is responsible for a loss in sensitivity in bioFETs with a charged surface. These experimental findings agree with the early theoretical works that find a low surface potential to be desirable for the best bioFET performance.

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