Subthreshold regime has the optimal sensitivity for nanowire FET biosensors

Abstract

Nanowire field-effect transistors (NW-FETs) are emerging as powerful sensors for detection of chemical/biological species with various attractive features including high sensitivity and direct electrical readout. Yet to date there have been limited systematic studies addressing how the fundamental factors of devices affect their sensitivity. Here we demonstrate that the sensitivity of NW-FET sensors can be exponentially enhanced in the subthreshold regime where the gating effect of molecules bound on a surface is the most effective due to the reduced screening of carriers in NWs. This principle is exemplified in both pH and protein sensing experiments where the operational mode of NW-FET biosensors was tuned by electrolyte gating. The lowest charge detectable by NW-FET sensors working under different operational modes is also estimated. Our work shows that optimization of NW-FET structure and operating conditions can provide significant enhancement and fundamental understanding for the sensitivity limits of NW-FET sensors.

Figure 1

Screening length effect on the operation and sensitivity of NW FET sensors. The working regime and effectiveness of gating effect induced by molecules at surface of NW-FET sensors are determined by the relative magnitude between carrier screening length λ_Si and nanowire size (radius) R. In the high carrier concentration regime where λ_Si≪R, NW-FET works in the linear regime, where the conductance G varies with gate voltage linearly. In the low carrier concentration regime where λ_Si≫R, NW-FET works in the depletion (subthreshold) regime where the G varies with gate voltage exponentially. In the linear regime, the field effect of positive/negative surface charges induces band bending and carrier depletion/enhancement inside the NW within a region of depth ∼ λ_Si. The amount of band bending at the NW surface is also denoted as surface potential shift Δϕ_Si. In the subthreshold (depletion) regime, carriers in NW have long screening length (λ_Si ≫R) and the field effect of surface charges can gate the whole NW, fully utilizing the high surface volume ratio of NW. In this case, the Fermi level E_F is shifted by Δϕ_Si relative to the band edges throughout the whole cross-section of NW.

Figure 2

pH sensing in the linear vs. subthreshold regime of a NW-FET. (a) Conductance G vs. electrolyte gate voltage V_g of a p-type silicon NW FET. The inset shows the schematic of electrolyte gating. This device has a trans-conductance ∼ 700 nS/V in the linear regime and subthreshold slope S ∼ 180 mV/decade in the subthreshold regime, with a threshold voltage V_T ∼ 0V. (b) Real time conductance data G(t) for pH sensing at V_g = −0.4 V (linear regime), 0 V (near threshold voltage) and 0.2 V (subthreshold regime). (c) Real time pH sensing data in (b) plotted as the percentage change, ΔG/G, with the conductance value at pH = 4 as reference point. In the subthreshold regime (V_g = + 0.2V), the device shows much larger percentage change in conductance as solution pH changes. (d) Device conductance as a function of pH value at V_g = −0.4, 0 and +0.2V. The blue and red lines are exponential and linear responses for pH induced surface potential shift of −30 mV/pH.

Figure 3

Sensing PSA/antibody conjugations by NW FET sensor: linear vs. subthreshold regime. (a) Schematic of PSA/PSA-Ab binding/unbinding equilibrium system. PSA-antibody molecules are first linked to nanowire surface. When there are PSA molecules present in sample solution, some antibody sites will be occupied by PSA. The binding of charged PSA molecules induces field gating effect which changes the device conductance. (b) Conductance vs. time data at electrolyte gate voltage V_g = 0, 0.15 and 0.45V when 15 pM PSA sample and buffer solution were sequentially delivered. The increased conductance between arrows was caused by binding of negatively charged PSA molecules on the p-type NW surface. (c) (Top panel) The absolute conductance change ΔG and relative conductance change ΔG/G vs V_g for sensing of 15 pM PSA. (Bottom panel) signal/noise ratio as a function of V_g for sensing 15pM PSA, which peaks at V_g = 0.45 V in the subthreshold regime before complete depletion of NW. (d) Electrolyte gating performance of this NW device with G in linear (left) or log (right) scale.

Figure 4

Greatly Improved PSA detection limit of NW FET sensor in the subthreshold regime (a)-(b) Conductance sensing of 15 pM and 0.75 pM PSA samples by a p-type NW FET sensor at linear regime (V_g = 0 V). The curves show a minimal PSA detection limit ∼ 0.75 pM for this device in the linear regime. (c)-(f) Conductance sensing of 15 pM, 0.75 pM, 37 fM and 1.5 fM PSA samples with device in the subthreshold regime (V_g = 0.45 V). It can be clearly seen that the minimal PSA detection limit is improved to ∼1.5 fM in the subthreshold regime of this device.