High-performance carbon nanotube field effect transistors with a thin gate dielectric based on a self-assembled monolayer

Individual single-walled carbon nanotube (SWCNT) field effect transistors (FETs) with a 2 nm thick silane-based organic self-assembled monolayer (SAM) gate dielectric have been manufactured. The FETs exhibit a unique combination of excellent device performance parameters. In particular, they operate with a gate-source voltage of only -1 V and exhibit good saturation, large transconductance, and small hysteresis (相似文献   

Current instability of carbon nanotube field effect transistors

The current instability of carbon nanotube field effect transistors (CNTFETs) is systematically studied under the influence of applied voltages, surfactants and temperatures. The devices were fabricated from carbon nanotubes and sodium dodecyl benzene sulfonate (SDBS) suspension using an ac dielectrophoresis (DEP) technique. The source and drain current for as-prepared p-type CNTFETs show an increase with time for the on-state, but a decrease for the off-state. Comparisons between constant and intermittent biasing conditions reveal that mobile ions could be the origin of the current instability. After removal of adsorbed SDBS, opposite transient behaviors of the current were observed, which can be attributed to the charge trapping induced screening effect.     

A carbon nanotube field effect transistor with a suspended nanotube gate

We investigate theoretically field effect transistors based on single-walled carbon nanotubes (CNTFET) and explore two device geometries with suspended multiwalled carbon nanotubes (MWNT) functioning as gate electrodes. In the two geometries, a doubly or singly clamped MWNT is electrostatically deflected toward the transistor channel, allowing for a variable gate coupling and leading to, for instance, a superior subthreshold slope. We suggest that the proposed designs can be used as nanoelectromechanical switches and as detectors of mechanical motion on the nanoscale.     

Decrease of the OFF state current of carbon nanotube field effect transistors via continuous repeated gate sweeping

Continuous repeated gate sweeping incorporated with substrate etching step is utilized to decrease the OFF state current in single-walled carbon nanotube field effect transistors with bundled carbon nanotubes. In particular, the effects of continuous repeated gate sweeping on transfer characteristic of transistors are examined. The etching step creates suspension in transistors at contact with metal electrodes as well as causing some single-walled carbon nanotubes to dramatically burn off by electrical current. By repeating gate sweeping, source-drain current gets smaller and smaller. This will eventually lead to the OFF state current less than 2 nA. Defects in the lattice of single-walled carbon nanotubes introduced by multiple gate sweeping could be the reason for this phenomenon. Contribution from possible hole trapping is also considered.     

High frequency scanning gate microscopy and local memory effect of carbon nanotube transistors

We use impedance spectroscopy to measure the high-frequency properties of single-walled carbon nanotube field effect transistors (swCN-FETs). Furthermore, we extend scanning gate microscopy (SGM) to frequencies up to 15 MHz and use it to image changes in the impedance of swCN-FET circuits induced by the SGM tip gate. In contrast to earlier reports, the results of both experiments are consistent with a simple RC parallel circuit model of the swCN-FET, with a time constant of 0.3 micros. We also use the SGM tip to show the local nature of the memory effect normally observed in swCN-FETs, implying that nanotube-based memory cells can be miniaturized to dimensions of the order of tens of nm.     

Piperidine induced polarity conversion in single-walled carbon nanotube field effect transistors

Piperidine is found to be an efficient electron doping agent that converts as-prepared p-type single-walled carbon nanotube (SWCNT) field effect transistors (FETs) into n-type SWCNT-FETs. Electron transfer from the amine group in piperidine to the SWCNTs is suggested to be the origin of the p- to n-type conversion. The effect of electron doping is further supported by the Raman tangential G(+) and G(-)-peak downshift up to 3 cm(-1) without the peak broadening. No detectable change in the Raman D-peak suggests non-covalent attachment of piperidine to the SWCNTs. A low temperature (110?°C) Si(3)N(4) passivation layer is used to maintain the long term air stability of the converted n-type devices. A complementary SWCNT inverter is demonstrated through integrating the n- and p-type SWCNT-FETs.     

Detection of tumor markers using single-walled carbon nanotube field effect transistors

We have developed a biosensor capable of detecting carcinoembryonic antigen (CEA) markers using single-walled carbon nanotube field effect transistors (SWNT-FETs). These SWNT-FETs were fabricated using nanotubes produced by a patterned catalyst growth technique, where the top contact electrodes were generated using conventional photolithography. For biosensor applications, SU-8 negative photoresist patterns were used as an insulation layer. CEA antibodies were employed as recognition elements to specific tumor markers, and were successfully immobilized on the sides of a single-walled carbon nanotube using CDI-Tween 20 linking molecules. The binding of tumor markers to these antibody-functionalized SWNT-FETs was then monitored continuously during exposure to dilute CEA solutions. The observed sharp decrease in conductance demonstrates the possibility of realizing highly sensitive, label-free SWNT-FET-based tumor sensors.     

Apta-biosensors for nonlabeled real time detection of human IgE based on carbon nanotube field effect transistors

In this study, we demonstrated the aptamer-based biosensor (apta-biosensor) using CNT-FET devices for label free detection of allergy diagnosis by IgE detection. In order to detect the IgE, two kinds of receptor (monoclonal IgE antibody and anti-IgE aptamer)-modified CNT-FET devices were fabricated. The binding event of the target IgE onto receptors was detected by monitoring the gating effect caused by the charges of the target proteins. Since the CNT-FET biosensors were used in buffer solution, it was crucial to use small-size receptors like aptamers than whole antibodies so that the charged target IgE could approach the CNT surface within the Debye length distance to give a large gating effect. The results show that CNT-FET biosensors using monoclonal IgE antibody had very low sensitivity (minimum detectable level 1000 ng/mL), while those based on anti-IgE aptamer could detect 50 ng/mL. Moreover, the aptamer-modified CNT-FET herein could successfully block non-target proteins and could selectively detect the target protein in an environment similar to human serum electrolyte. Therefore, aptamer-based CNT-FET devices enable the production of label-free ultrasensitive electronic biosensors to detect clinically important biomarkers for disease diagnosis.     

Local-gated single-walled carbon nanotube field effect transistors assembled by AC dielectrophoresis

We present a simple and scalable technique for the fabrication of solution processed and local-gated carbon nanotube field effect transistors (CNT-FETs). The approach is based on the directed assembly of individual single-walled carbon nanotubes from dichloroethane via AC dielectrophoresis (DEP) onto pre-patterned source and drain electrodes with a local aluminum gate in the middle. Local-gated CNT-FET devices display superior performance compared to a global back gate with on-off ratios >10(4) and maximum subthreshold swings of 170?mV/dec. The local bottom-gated DEP-assembled CNT-FETs will facilitate large-scale fabrication of complementary metal-oxide-semiconductor (CMOS) compatible nanoelectronic devices.     

Fabrication of Schottky barrier carbon nanotube field effect transistors using dielectrophoretic-based manipulation

Nanoscale electronic devices made from carbon nanotubes (CNTs) such as transistors and sensors are much smaller and potentially more versatile than those built using conventional IC technology. In this paper, we present a method that uses dielectrophoretic (DEP) manipulation process for the fabrication of single-channel and multi-channel carbon nanotube field effect transistors (CNT-FETs). For a typical fabrication process, single-walled carbon nanotubes (SWCNTs) are first pre-aligned to micron-precision range between two microelectrodes using DEP technique. The typically applied alternating current (AC) voltage to generate the DEP force for manipulation has a frequency of 1 MHz and amplitude of 10 V. We first demonstrated single-channel or multi-channel structures of CNT-FETs. An AFM is then used to "clean" or "sweep away" unwanted particles or CNTs around the electrodes. Lastly, the fabricated FETs were covered in a polymethylmethacrylate (PMMA) thin film and treated with an annealing process. The PMMA covered devices show improved performances over the non-covered devices.     

Assessment of high-frequency performance potential of carbon nanotube transistors

Self-consistent quantum simulations are used to explore the high-frequency performance potential of carbon nantube field-effect transistors (CNTFETs). The cutoff frequency expected for a recently reported CNT Schottky-barrier FET is well below the performance limit, due to the large parasitic capacitance between electrodes. We show that using an array of parallel nanotubes as the transistor channel reduces parasitic capacitance per tube. Increasing tube density gives a large improvement of high-frequency performance when tubes are widely spaced and parasitic capacitance dominates but only a small improvement when the tube spacing is small and intrinsic gate capacitance dominates. Alternatively, using quasi-one-dimensional nanowires as source and drain contacts should significantly reduce parasitic capacitance and improve high-frequency performance. Ballistic CNTFETs should outperform ballistic Si MOSFETs in terms of the high-frequency performance limit because of their larger band-structure-limited velocity.     

Label-free protein biosensor based on aptamer-modified carbon nanotube field-effect transistors

We have fabricated label-free protein biosensors based on aptamer-modified carbon nanotube field-effect transistors (CNT-FETs) for the detection of immunoglobulin E (IgE). After the covalent immobilization of 5'-amino-modified 45-mer aptamers on the CNT channels, the electrical properties of the CNT-FETs were monitored in real time. The introduction of target IgE at various concentrations caused a sharp decrease in the source-drain current, and a gradual saturation was observed at lower concentrations. The amount of the net source-drain current before and after IgE introduction on the aptamer-modified CNT-FETs increased as a function of IgE concentration. The detection limit for IgE was determined as 250 pM. We have also prepared CNT-FET biosensors using a monoclonal antibody against IgE (IgE-mAb). The electrical properties of the aptamer- and antibody-modified CNT-FETs were compared. The performance of aptamer-modified CNT-FETs provided better results than the ones obtained using IgE-mAb-modified CNT-FETs under similar conditions. Thus, we suggest that the aptamer-modified CNT-FETs are promising candidates for the development of label-free protein biosensors.     

A general approach for high yield fabrication of CMOS-compatible all-semiconducting carbon nanotube field effect transistors

We report strategies to achieve both high assembly yield of carbon nanotubes at selected positions of the circuit via dielectrophoresis (DEP) and field effect transistor (FET) yield using an aqueous solution of semiconducting-enriched single-walled carbon nanotubes (s-SWNTs). When the DEP parameters were optimized for the assembly of individual s-SWNTs, 97% of the devices showed FET behavior with a maximum mobility of 210 cm2 V(-1) s(-1), on-off current ratio ～10(6) and on-conductance up to 3 μS, but with an assembly yield of only 33%. As the DEP parameters were optimized so that one to five s-SWNTs are connected per electrode pair, the assembly yield was almost 90%, with ～90% of these assembled devices demonstrating FET behavior. Further optimization gave an assembly yield of 100% with up to 10 SWNTs per site, but with a reduced FET yield of 59%. Improved FET performance including higher current on-off ratio and high switching speed were obtained by integrating a local Al2O3 gate to the device. Our 90% FET with 90% assembly yield is the highest reported so far for carbon nanotube devices. Our study provides a pathway which could become a general approach for the high yield fabrication of complementary metal oxide semiconductor (CMOS)-compatible carbon nanotube FETs.     

Carbon nanotube field effect transistors with suspended graphene gates

Novel field effect transistors with suspended graphene gates are demonstrated. By incorporating mechanical motion of the gate electrode, it is possible to improve the switching characteristics compared to a static gate, as shown by a combination of experimental measurements and numerical simulations. The mechanical motion of the graphene gate is confirmed by using atomic force microscopy to directly measure the electrostatic deflection. The device geometry investigated here can also provide a sensitive measurement technique for detecting high-frequency motion of suspended membranes as required, e.g., for mass sensing.     

Electrical transport properties of single wall carbon nanotube/polyurethane composite based field effect transistors fabricated by UV-assisted direct-writing technology

We report on the fabrication and transport properties of single-walled carbon nanotube (SWCNT)/polyurethane (PU) nanocomposite microfiber-based field effect transistors (FETs). UV-assisted direct-writing technology was used, and microfibers consisting of cylindrical micro-rods, having different diameters and various SWCNT loads, were fabricated directly onto SiO?/Si substrates in a FET scheme. The room temperature dc electrical conductivities of these microfibers were shown to increase with respect to the SWCNT concentrations in the nanocomposite, and were about ten orders of magnitude higher than that of the pure polyurethane, when the SWCNT load ranged from 0.1 to 2.5 wt% only. Our results show that for SWCNT loads ≤ 1.5 wt%, all the microfibers behave as a FET with p-type transport. The resulting FET exhibited excellent performance, with an I(on)/I(off) ratio of 10? and a maximum on-state current (I(on)) exceeding 70 μA. Correlations between the FET performance, SWCNTs concentration, and the microfiber diameters are also discussed.     

High yield assembly and electron transport investigation of semiconducting-rich local-gated single-walled carbon nanotube field effect transistors

We report the fabrication and electron transport investigation of individual local-gated single-walled carbon nanotube field effect transistors (SWNT-FET) with high yield using a semiconducting-rich carbon nanotube solution. The individual semiconducting nanotubes were assembled at the selected position of the circuit via dielectrophoresis. Detailed electron transport investigations on 70 devices show that 99% display good FET behavior, with an average threshold voltage of 1 V, subthreshold swing as low as 140 mV/dec, and on/off current ratio as high as 8 × 10(5). The high yield directed assembly of local-gated SWNT-FET will facilitate large scale fabrication of CMOS (complementary metal-oxide-semiconductor) compatible nanoelectronic devices.     

Active properties of carbon nanotube field-effect transistors deduced from S parameters measurements

AC performances of carbon nanotube field-effect transistors (CNT-FETs) are analyzed by means of scattering parameters measurements. The active ac properties of CNT-FETs are clearly demonstrated up to 80 MHz and indications of active behavior are obtained up to 1 GHz. From these measurements, a small signal equivalent circuit is proposed and validated up to 10 MHz. The extraction procedure and the determination of the intrinsic ac elements of CNT-FETs are pointed out.     

Local gate effect of mechanically deformed crossed carbon nanotube junction

In this work, we have demonstrated that the local deformation at the crossed carbon nanotube (CNT) junctions can introduce significant tunable local gate effect under ambient environment. Atomic force microscope (AFM) manipulation of the local deformation yielded a variation in transconductance that was retained after removing the AFM tip. Application of a large source-drain voltage and pressing the CNT junction above a threshold pressure can respectively erase and recover the transconductance modulation reversibly. The local gate effect is found to be independent of the length of the crossed CNT and attributed to the charges residing at the deformed junctions due to formation of localized states. The number of localized charges is estimated to be in the range of 10(2) to 10(3). These results may find potential applications in electromechanical sensors and could have important implications for designing nonvolatile devices based on crossed CNT junctions.