Comprehensive Understanding of Silicon-Nanowire Field-Effect Transistor Impedimetric Readout for Biomolecular Sensing

Abstract

Impedance sensing with silicon nanowire field-effect transistors (SiNW-FETs) shows considerable potential for label-free detection of biomolecules. With this technique, it might be possible to overcome the Debye-screening limitation, a major problem of the classical potentiometric readout. We employed an electronic circuit model in Simulation Program with Integrated Circuit Emphasis (SPICE) for SiNW-FETs to perform impedimetric measurements through SPICE simulations and quantitatively evaluate influences of various device parameters to the transfer function of the devices. Furthermore, we investigated how biomolecule binding to the surface of SiNW-FETs is influencing the impedance spectra. Based on mathematical analysis and simulation results, we proposed methods that could improve the impedimetric readout of SiNW-FET biosensors and make it more explicable.

Keywords: biosensors; impedimetric readout; silicon-nanowire field-effect transistor; simulation program with integrated circuit emphasis (SPICE); transistor transfer function.

Figure 1

(a) Optical image, SEM image and cross-section view of the SiNW-FET arrays used in the simulation model, (b) Electrically equivalent circuit for the SiNW-FET including a transistor, parameters of source and drain contacts and of the electrolyte; (c) ac small-signal model for the SiNW ISFET with the readout operational amplifier, (d) magnitude and phase plots for the TTF in presence of the parasitic capacitances C_ps and C_pd.

Figure 2

Effect of drain and source capacitances of the SiNW-FET as well as electrolyte parameters to the frequency spectrum. TTF spectrum of the SiNW-FET with varying source capacitance (C_ps) for two scenario of drain capacitance with C_pd = 0 fF (a) and C_pd = 33 fF (b). (c) Effect the drain capacitance (C_pd) while the source capacitance is equal to 50 fF. As C_pd is introduced, a peak appears in the TTF spectra. An increase in C_pd leads to a change in the amplitude and in the position of the peak. (d) Effect of conductivity of electrolyte on the TTF spectra. The peak amplitude decreases with the decrease of electrolyte concentration.

Figure 3

(a) Magnitude spectrum of the SiNW-FET sensor with t_ox varying as 6 nm, 12 nm and 24 nm. The effect of the oxide thickness causes a change in the TTF spectrum at low frequencies. (b) Magnitude spectrum with pH of electrolyte varying from 3.0 to 11.0.

Figure 4

Magnitude spectrum with varying feedback parameters of the readout transimpedance amplifier. Change of the feedback capacitance (a) and feedback resistance (b) cause change in the amplitude and the position of the peak in the TTF spectra of SiNW-FET sensors.

Figure 5

(a) Schematic of the DNA detection with a SiNW-FET considering the biomolecules only at the gate, while neglecting the interaction of the biomolecules with the drain and the source. (b) Electrically equivalent circuit for this measurement including the additional RC circuit elements caused by biomolecular layer on the gate. Magnitude spectrum of SiNW-FET with t_bio varying from 1 to 10 nm in case C_pd = 0 fF (c) and C_pd = 33 fF (d), while other parameters stay constant.

Figure 6

Magnitude spectrum of transconductance of a SiNW-FET for various biomolecular immobilizations [27]. The image has been reproduced with permission from [27].

Figure 7

(a) Schematic of the DNA detection with a SiNW-FET taking the attachment of a biomolecular layer to the contact lines in to account as well. (b) Electrically equivalent circuit for this measurement including the additional RC circuit elements in red. (c) Magnitude spectrum with t_bio varying from 1 to 10 nm showing much stronger changes in the spectrum.

Figure 8

Magnitude spectrum with t_bio varying from 1–10 nm, when we assume only binding of biomolecules inside a microfluidic structure. (a) Only small portions of each of the source and drain contact lines (1 μm × 10 μm) are exposed to biomolecules, which minimizes the effect of the parasitic capacitances. (b,c) Change in the impedance spectra in this case is mainly caused by the binding of the biomolecules on the SiNW-FET gate.