Multi-mode humidity sensitivity of carbon nanotubes field-effect transistors
Résumé
Single-walled carbon nanotube-based field effect transistors have demonstrated highly promising gas sensing properties, especially a particularly strong sensitivity to humidity. The sensitivity to humidity impairs the use of the transistors in logic circuits, while the devices are usually very unstable as humidity sensors. The humidity dependence of electrical characteristics is generally attributed to charge trapping and water adsorption either on the oxide layer or on the nanotubes surface, but a detailed understanding of the mechanisms is prevented by the diversity of behaviors reported throughout the literature as well as by the response variability observed even with batch-fabricated samples [1]. We propose here a novel analysis of the different modes of humidity sensitivity in CNTFET based on a systematic, extensive multiparameter study of the electrical response of a series of CNTFET devices. We consider transistors whose channel consist in a network of mostly semiconducting, in-place CVD-grown SWNTs directly connecting source and drain electrodes [2], and we monitor systematically the characteristics of the transfer curves with respect to humidity in a metrological approach (Imax, Imin, threshold voltage, hysteresis voltage, subthreshold slope...). The significant variability of responses is interpreted using a compact-model-based approach [3]: we introduce a humidity-modulated voltage difference between effective gate voltage along the nanotube and imposed gate voltage. Using this approach, we rigorously interpret the variation of all the parameters while confirming the existence of two regimes, one at low humidity involving water molecules adsorption around charge trap, the other at high humidity involving a charging effect in the full layer of water molecules adsorbed on the oxide surface.