Browsing by Subject "Field effect transistor"
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Item Electron transport in graphene transistors and heterostructures : towards graphene-based nanoelectronics(2012-05) Kim, Seyoung, 1981-; Banerjee, Sanjay; Tutuc, Emanuel, 1974-; MacDonald, Allan; Dodabalapur, Ananth; Lee, Jack C.; Register, Leonard F.Two graphene layers placed in close proximity offer a unique system to investigate interacting electron physics as well as to test novel electronic device concepts. In this system, the interlayer spacing can be reduced to value much smaller than that achievable in semiconductor heterostructures, and the zero energy band-gap allows the realization of coupled hole-hole, electron-hole, and electron-electron two-dimensional systems in the same sample. Leveraging the fabrication technique and electron transport study in dual-gated graphene field-effect transistors, we realize independently contacted graphene double layers separated by an ultra-thin dielectric. We probe the resistance and density of each layer, and quantitatively explain their dependence on the backgate and interlayer bias. We experimentally measure the Coulomb drag between the two graphene layers for the first time, by flowing current in one layer and measuring the voltage drop in the opposite layer. The drag resistivity gauges the momentum transfer between the two layers, which, in turn, probes the interlayer electron-electron scattering rate. The temperature dependence of the Coulomb drag above temperatures of 50 K reveals that the ground state in each layer is a Fermi liquid. Below 50 K we observe mesoscopic fluctuations of the drag resistivity, as a result of the interplay between coherent intralayer transport and interlayer interaction. In addition, we develop a technique to directly measure the Fermi energy in an electron system as a function of carrier density using double layer structure. We demonstrate this method in the double layer graphene structure and probe the Fermi energy in graphene both at zero and in high magnetic fields. Last, we realize dual-gated bilayer graphene devices, where we investigate quantum Hall effects at zero energy as a function of transverse electric field and perpendicular magnetic field. Here we observe a development of v = 0 quantum Hall state at large electric fields and in high magnetic fields, which is explained by broken spin and valley spin symmetry in the zero energy Landau levels.Item Graphene field effect transistors for high performance flexible nanoelectronics(2014-05) Lee, Jongho, active 21st century; Akinwande, DejiDespite the widespread interest in graphene electronics over the last decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this work, we investigate the realization of high-performance graphene field effect transistors implemented on flexible plastic substrates. The optimum device structure for high-mobility and high-bendability is suggested with experimental comparison among diverse structures including top-gate GFETs (TG-GFETs), single/multi-finger embedded-gate GFETs with high-k dielectrics (EG-highk/GFETs), and embedded-gate GFETs with hexagonal boron nitride (h-BN) dielectrics. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobility of 8,000 cm²/V·s, and importantly 32 GHz cut-off frequency. Mechanical studies reveal robust transistor performance under repeated bending down to 0.7 mm bending radius whose tensile strain corresponds to 8.6%. Passivation techniques, with robust mechanical and chemical protection in order to operate under harsh environments, for embedded-gate structures are also covered. The integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene.Item Inkjet printed single-walled carbon nanotube field effect transistors(2016-05) Jang, Seonpil; Dodabalapur, Ananth, 1963-; Akinwande, Deji; Chen, Ray; Ho, Paul; Yu, GuihuaInkjet printing technology has the potential to drastically reduce the process time and cost by generating the patterns without physical masks and conventional vacuum processes. In addition, the inkjet printing process can be applied to flexible and large area substrates. Among the printable semiconductors, single walled carbon nanotubes (SWCNTs) have been attracting increasing attention for their high carrier mobility and potential application in transparent, flexible, high- current and high frequency electronics. The effects of fluoropolymer capping onto SWCNT devices are investigated. Remarkable improvements in key device characteristics of SWCNT field-effect transistors (FETs) are achieved by coating of the active semiconductor with a fluoropolymer layer such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). These favorable changes in device characteristics also enhance circuit performance. The origins of these improvements are the dipolar nature of the fluoropolymer and the mechanism is confirm by exposing SWCNT FETs to a number of vapor phase polar molecules which produce similar effects on the FET characteristics as the application of P(VDF-TrFE). High-performance inkjet printed single walled carbon nanotube (SWCNT) field effect transistors (FETs) with channel lengths of 150-250 nm are demonstrated. Optimized electrode geometry has been developed to confine the inkjet droplet to the active channel area. This minimizes waste of material outside of the channel while enabling short channel length devices that exhibit high effective carrier mobilities and transconductances. This novel fabrication approach is compatible with roll-to-roll processing and enables the formation of high-performance short channel device arrays based on inkjet printing with at least a 50-fold reduction in consumption of semiconducting SWCNT ink compared to other solution processing methods. In these short channel SWCNT FETs, the influences of nanotube bending and gate insulator-semiconductor interface modification on the characteristics of inkjet printed short channel length SWCNT are investigated. Employing recessed source and drain (S/D) electrodes to minimize the mechanical distortion of CNTs, high performance short channel ambipolar transistors based on inkjet printed SWCNTs are demonstrated. Mechanical distortion of the nanotubes due to bending near source and drain contacts when they are not recessed is found to suppress electron transport and transform the ambipolar transistors into p-type devices. Inclusion of interfacial polymer layers such as P(VDF-TrFE) between the SWCNTs and Al2O3 top dielectric also results in p-type doping and reductions in electron transport transforming amibipolar transistors into p-type devices.