Optimal signal-to-noise ratio for silicon nanowire biochemical sensors

Abstract

The signal-to-noise ratio (SNR) for silicon nanowire field-effect transistors operated in an electrolyte environment is an essential figure-of-merit to characterize and compare the detection limit of such devices when used in an exposed channel configuration as biochemical sensors. We employ low frequency noise measurements to determine the regime for optimal SNR. We find that SNR is not significantly affected by the electrolyte concentration, composition, or pH, leading us to conclude that the major contributions to the SNR come from the intrinsic device quality. The results presented here show that SNR is maximized at the peak transconductance. Silicon nanowire field-effect transistors (SiNW-FET) have shown great sensitivity when employed as biological/ chemical sensors (bioFETs). [1][2][3] The principle of operation is that a charged species bound to the nanowire (NW) surface (modified with some receptor molecules) induces a change in surface potential at the NW surface, which translates into a change in drain-to-source current (DI) which is then measured. It is well-known that the sensitivity (defined as DI/I for a current based sensing experiment) is maximized in the subthreshold regime. 4-6 However, it is also known that the normalized current noise power amplitude (S I /I 2 ) reaches a plateau and is highest in the subthreshold regime for siliconsilicon oxide devices 7,8 and concerns have been expressed that signal-to-noise ratio (SNR) would be impacted for measurements carried out in subthreshold. 4,9 On the other hand, S I /I 2 is lower in the linear regime but the sensitivity is also lower. In order to determine the ideal regime for optimal SNR, we carry out both I-V and noise measurements for solution gated devices. Our measurements indicate that the current noise is independent of electrolyte concentration, composition, or pH, leading us to conclude that the intrinsic electronic properties of the SiNW bioFETs determine the optimal SNR achievable by these sensors. We also find that SNR is maximized in the linear regime at the point where the transconductance is largest.The SiNWs were fabricated from SOI wafers (Soitec) with a high resistivity boron doped active layer (>2000 ohm cm) as described previously. 10 Devices used for the experiments were nominally 100 nm wide and 5 lm long. The devices were covered with a passivation layer of SU-8 (an epoxy based negative photoresist) with windows opened for the NW channel and the contact pads. An optical micrograph of the devices is shown in Fig. 1(b). The NW surfaces were functionalized with a monolayer of APTES (3-aminopropyltriethoxysilane) using the protocol described earlier, 11 which increases device stability and reduces gate leakage current in solution. A fluidic well was then glued on top of each chip for the sensing experiments, with a platinum wire used as the solution gate electrode. The device structure and experimental setup is shown schematically in Fig. 1(a). For all noise and transfer characteristics measurements, the back-gate contact was l...

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