Signal-to-noise ratio in dual-gated silicon nanoribbon field-effect sensors

Abstract

Recent studies on nanoscale field-effect sensors reveal the crucial importance of the low frequency noise for determining the ultimate detection limit. In this letter, the 1/f -type noise of Si nanoribbon field-effect sensors is investigated. We demonstrate that the signal-to-noise ratio can be increased by almost two orders of magnitude if the nanoribbon is operated in an optimal gate voltage range. In this case, the additional noise contribution from the contact regions is minimized, and an accuracy of 0.5 ‰ of a pH shift in one Hz bandwidth can be reached. showed that the SNR increases in the subthreshold regime, which is therefore the preferred regime for high sensitivity. However, a more detailed understanding of the noise properties is needed to optimize the SNR across the full operating range of the FET.In the present work, we measure the low-frequency 1/f noise of a dual-gated NR-FET 17,19-21 in ambient and in a buffer solution and determine the resolution limit expressed in a noise equivalent threshold voltage shift δV th , the latter being the measurement quantity in these types of sensors. We identify two regimes which differ in the relative importance of the contact and intrinsic NR resistance. The lowest value in δV th is found when the working point of the NR-FET is adjusted such that the intrinsic NR resistance dominates.In the other case when the contact resistance dominates the noise can be larger by almost two orders of magnitude for nominally the same overall resistance. This result shows the importance of being able to adjust the operating point properly. In the best possible case we determine a resolution limit of 0.5 ‰ of a pH change in one Hz bandwidth, which is comparable to a commercial pH meter (0. This high-yield process provides reproducible, hysteresis-free FETs with the following dimensions: length × width × height = 10 µm × 700 nm × 80 nm (Fig. 1a). A thin Al 2 O 3 layer was deposited on the device to ensure leakage-free operation in an electrolyte solution. In addition, a liquid channel was formed in a photoresist layer, reducing the total area exposed to the electrolyte. which on its own is more strongly capacitively coupled to the liquid than to the back-gate.We refer to the two regimes as contact and NR dominated. The material dependent parameter α accounts for scattering effects and the constant N denotes the number of fluctuators in the system.In Fig. 3, the normalized noise amplitude S V /V Here, we have made use of the relation δG/G = √ S V /V sd .In Fig. 4a we show δV th when the controlling gate is V bg for data measured in air ( ) together with the data acquired in buffer solution at V ref = −0.3 V ( ). Both curves show a very similar behavior. Since we know that the liquid data obtained atcontact dominated, we conclude that the measurement in air is also contact dominated.In Fig. 4b we summarize δV th for measurements done in an electrolyte. To obtain δV th , we consistently use V lg as the controlling gate for all three data sets in this figure. Interestingly, ...

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