Preparation And Performance Study Of The Carbon Nanotube Network Structure Field Effect Transistor

Single-walled carbon nanotubes (SWNTs) are unique quasi one-dimensional structures with remarkable properties. Dictated by their chirality, SWNTs can be electrically semi-conducting (s-) or metallic (m-). The m-SWNTs are contaminates for field-effect transistors (FETs) since their presence defeats the routine fabrication of FETs with high ON-state current and large ON/OFF current ratio based on individual s-SWNTs for nanoelectronics applications. FETs based on thin films composed of large amounts of SWNTs in form of random networks, i.e., CNN, are attractive since the property of the thin films is statistically determined by a collective effort of the SWNTs and the adverse effect of m-SWNTs is thus minimized. Such CNNFETs can still achieve outstanding performance as a switching element, with decent ON-state current and ON/OFF current ratio. In addition, the solution-phase processing of a CNN is compatible with the printing technology, which makes it possible to fabricate electronic devices and circuits on flexible substrates. Therefore, FETs based on CNNs have recently become a research focus especially for logic devices in low-cost and large-area electronics.This thesis work starts from the preparation of an SWNT suspension. The addition of a surfactant, here sodium dodecylbenzene sulfonate (SDBS), is firstly studied with a focus on the dispersibility of SWNTs in an aqueous solution. The results show that for an SDBS concentration of 1%, sonication for 30 minutes, centrifugation at 16000 revolutions per minutes for 3 hours are the most appropriate parameters to use in our experiment. The suspension produced this manner is then drop-cast onto the channel region of a back-gate FET with pre-defined source and drain electrodes made of palladium on titanium bi-layers. The performances of the fabricated devices are investigated by means of atomic force microscopy (AFM), electrical characterization on a probe-station equipment with a parameter analyzer, Raman spectrum and so on. In this thesis, current hysteresis of the CNNFETs constitutes a special case study for its great importance for circuit applications. The physical origin of the hysteresis is explored, and the electron capture and release by the traps close to the SWNTs under gate bias are identified as the cause. A pulsed-voltage method is proposed to reduce the hysteresis. The relaxation of trapped electrons upon a pulsed gate voltage is accounted for by assuming both a fast and a slow exponential decay occurring simultaneously. Finally, SWNTs are incorporated in an organic semiconducting polymer, poly(3-hexylthiophene) (P3HT), to make a composite suspension. Spectral analysis of absorption shows that P3HT disperses SWNTs with its thiophene ring structure while the presence of SWNTs increases the conjugation length of P3HT. As a result, the electron mobility is enhanced. The composite suspension is spin-coated onto the channel region of a FET. Well-performing organic thin-film transistors are thus achieved, with an ON/OFF current ratio stable at more than two orders of magnitude.