Evaluating Uniformity, Contact Geometries, and Size Scaling in Carbon Nanotube Thin-Film Transistors

From display-driving transistors to biosensors, semiconducting carbon nanotube (CNT) thin films have many potential use cases. While recent advances in solution-processable CNTs have made them more attainable, the performance of CNT thin-film devices is often limited by variability and resistance at the contact interfaces. In this work, we used statistical distributions of key performance metrics from CNT thin-film transistors (TFTs) to gain insights into: (1) the influence of different contact geometries; (2) the impact of scaling toward submicron dimensions; and (3) the uniformity of nanotube networks deposited by a facile dip-coating technique. By mapping the spatial uniformity of TFTs across a chip, the relative impact of CNT density variation on device performance was determined and systematic improvements were made through modifications to the device geometry. A simple self-aligned process was developed for forming electrical contacts on the exposed ends of carbon nanotubes - "edge contacts" - and these exhibited improved reliability and overall distributions of performance metrics compared to more traditional sidewall contacts. Through scaling the TFT channel and contact lengths, it was discovered that no contact geometries were significantly contact-length-limited above dimensions of ∼50 nm, yet statistical on- and off-state differences emerged in micrometer-scale devices, showing contact geometry can have relevance even for relatively large TFTs. Additional insights were gained into the limitations of patterned plasma etching of CNT thin films and the gate dependence of contact resistance in different geometries. Overall, these findings reveal the importance of device geometry when considering the uniformity and scalability of CNT-TFTs into the submicron regime.