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.     

Study of carbon nanotube field effect transistors performance based on changes in gate parameters

Carbon nanotubes are known as an interesting material to be used in the next generations of electronic technology, especially at nano regime. Nowadays, carbon nanotube field effect transistor or CNTFET is one of the promising devices for future electronic applications. A CNTFET which uses carbon nanotube as channel or source/drain region is the most promising candidate for replacing the current silicon transistor technology. The study of modern manufacturing approach and impact of device parameters on its performance is one of the important research fields in nanoelectronics. In this paper we study some aspects of changes in gate parameters at different channel diameters. This paper shows that for small values of diameter, increasing the dielectric constant of gate insulator doesn't help to improve the performance as value of dielectric constant of gate insulator reaches a certain amount. Also, increasing the oxide thickness of gate insulator doesn't always decrease transistor performance. For high diameter values, increasing the thickness up to a certain value improves the transistor performance.     

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.     

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   

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.     

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.     

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.     

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.     

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.     

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.     

Length scaling of carbon nanotube transistors

Carbon nanotube field-effect transistors are strong candidates in replacing or supplementing silicon technology. Although theoretical studies have projected that nanotube transistors will perform well at nanoscale device dimensions, most experimental studies have been carried out on devices that are about ten times larger than current silicon transistors. Here, we show that nanotube transistors maintain their performance as their channel length is scaled from 3 μm to 15 nm, with an absence of so-called short-channel effects. The 15-nm device has the shortest channel length and highest room-temperature conductance (0.7G?) and transconductance (40 μS) of any nanotube transistor reported to date. We also show the first experimental evidence that nanotube device performance depends significantly on contact length, in contrast to some previous reports. Data for both channel and contact length scaling were gathered by constructing multiple devices on a single carbon nanotube. Finally, we demonstrate the performance of a nanotube transistor with channel and contact lengths of 20 nm, an on-current of 10 μA, an on/off current ratio of 1 x 10?, and peak transconductance of 20 μS. These results provide an experimental forecast for carbon nanotube device performance at dimensions suitable for future transistor technology nodes.     

Radiation hardness of the electrical properties of carbon nanotube network field effect transistors under high-energy proton irradiation

The effect of high-energy proton irradiation on the physical properties of carbon nanotubes (CNTs) was investigated. The focus of the study was on the electrical properties of single-walled carbon nanotube (SWNT) network devices exposed to proton beams. Field-effect transistors (FETs) of network type were fabricated using SWNTs and were then irradiated by high-energy proton beams of 10-35?MeV with a fluence of 4 × 10(10)-4 × 10(12)?cm(-2) that are comparable to the aerospace radiation environment. The electrical properties of both metallic and semiconducting CNT network FET devices underwent no significant change after the high-energy proton irradiation, indicating that the CNT network devices are very tolerant in proton beams. Raman spectra confirm the proton-radiation hardness of CNT network FET devices. The radiation hardness of CNT network FET devices promises therefore the potential usefulness of CNT-based electronics for future space application.     

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.     

Novel ring structure for minimisation of screening effect in carbon nanotube based field emitters

Carbon Nanotubes (CNTs) are promising candidates for cold cathodes because of their high aspect ratio and robustness. However, the major hindrance in cold cathode based applications is the screening effect, which reduces the effective field at the tip and thereby the current density. The emission current can be improved by minimising the screening effect. The adverse effect of screening can be addressed by either controlling the growth density or by optimising the patterns of CNT cathodes. Here, novel patterns have been used to increase edge length per unit area in planar vertically aligned CNT bundles. Our motive was to increase the number of effective emitters, since the CNT at the edges are less screened by the proximal CNTs. By varying geometry and spacing of solid CNT dot patterns and by introducing the square ring structures; we could successfully enhance the effective emitters at the edges. It has been observed that an enhancement of edge length from 0.032 per micron to 0.2 per micron increases the current density from 0.71mA/cm² to 16.2 mA/cm² at a field of 4.5 V/μm. CNTs in dotted structure with high value of edge length per unit area emit very high current density as compared to other dotted structures with low value of edge length per unit area Simulation studies confirms our argument that CNTs at the corners are the least screened and have the maximum local electric field.     

Lithography-free fabrication of carbon nanotube network transistors

A novel non-lithographic technique for the fabrication of carbon nanotube thin film transistors is presented. The whole transistor fabrication process requires only one mask which is used both to pattern transistor channels based on aerosol synthesized carbon nanotubes and to deposit electrodes by metal evaporation at different angles. An important effect of electrodynamic focusing was utilized for the directed assembly of transistor channels with feature sizes smaller than the mask openings. This dry non-lithographic method opens up new avenues for device fabrication especially for low cost flexible and transparent electronics.     

Three-dimensional electrostatic effects of carbon nanotube transistors

We explore the three-dimensional (3-D) electrostatics of planar-gate carbon nanotube field-effect transistors (CNTFETs) using a self-consistent solution to the Poisson equation with equilibrium carrier statistics. We examine the effects of the gate insulator thickness and dielectric constant and the source/drain contact geometry on the electrostatics of bottom-gated (BG) and top-gated (TG) devices. We find that the electrostatic scaling length is mostly determined by the gate oxide thickness, not by the oxide dielectric constant. We also find that a high-k gate insulator does not necessarily improve short-channel immunity because it increases the coupling of both the gate and the source/drain contact to the channel. It also increases the parasitic coupling of the source/drain to the gate. Although both the width and the height of the source and drain contacts are important, we find that for the BG device, reducing the width of the 3-D contacts is more effective for improving short channel immunity than reducing the height. The TG device, however, is sensitive to both the width and height of the contact. We find that one-dimensional source and drain contacts promise the best short channel immunity. We also show that an optimized TG device with a thin gate oxide can provide near ideal subthreshold behavior. The results of this paper should provide useful guidance for designing high-performance CNTFETs.     

Chemical optimization of self-assembled carbon nanotube transistors

We present the improvement of carbon nanotube field effects transistors (CNTFETs) performances by chemical tuning of the nanotube/substrate and nanotube/electrode interfaces. Our work is based on a method of selective placement of individual single walled carbon nanotubes (SWNTs) by patterned aminosilane monolayer and its use for the fabrication of self-assembled nanotube transistors. This method brings a relevant solution to the problem of systematic connection of self-organized nanotubes. The aminosilane monolayer reactivity can be used to improve carrier injection and doping level of the SWNT. We show that the Schottky barrier height at the nanotube/metal interface can be diminished in a continuous fashion down to an almost ohmic contact through these chemical treatments. Moreover, sensitivity to 20 ppb of triethylamine is demonstrated for self-assembled CNTFETs, thus opening new prospects for gas sensors taking advantages of the chemical functionality of the aminosilane used for assembling the CNTFETs.