Link among array non-uniformity, threshold voltage, and subthreshold swing degradation in aligned array carbon nanotube field effect transistors

Abstract

Threshold voltage variation is a problem in field effect transistors (FETs) fabricated from arrays of aligned semiconducting carbon nanotubes. Moreover, the subthreshold swing of FETs fabricated from arrays of nanotubes is often several times larger than the subthreshold swing of FETs fabricated from a single nanotube or nanotube bundle. Here, we experimentally quantify the impact of nanotube bundling and array non-uniformity on threshold voltage variation and subthreshold swing degradation in arrays of aligned semiconducting nanotubes deposited by floating evaporative self-assembly. The height (and, therefore, degree of bundling) and array morphology of more than one hundred back-gated FETs (with the nanotube channel exposed) are mapped via atomic force microscopy. Both threshold voltage and subthreshold swing significantly increase as the degree of bundling increases—showing a clear link between the physical characteristics of an array and electrical properties. Modeling shows that much of the subthreshold swing degradation in arrays can be attributed to the bundling–threshold voltage link, in which polydispersity in bundle size within an array causes a bundle-by-bundle shift in threshold voltage and a gradual turn-on of the FET. Increased off-current also plays a role in increasing the sub-threshold swing. These findings highlight the importance of synthesizing and assembling arrays of aligned nanotubes that are highly ordered, with uniform and repeatable morphology.