Carbon Nanotube Complementary Wrap-Gate Transistors

Drain current vs gate voltage and drain current vs drain voltage characteristics have been derived in a ferroelectric gate-all-around carbon nanotube (CNT) transistor. An approximate analysis to derive the polarization characteristics in a cylindrical ferroelectric capacitor is first presented. It has been found that the characteristics can be approximated by those of a parallel-plate capacitor having the same area as the inner electrode of the cylindrical capacitor and the same thickness as the ferroelectric cylinder. Then, the drain current characteristics of the ferroelectric gate-all-around CNT transistor are derived by combining the analysis on the cylindrical capacitor and the ballistic transport theory on CNT transistors. The gate structure is assumed to be such that CNTs with diameters of 1 and 2 nm are surrounded with a 5-nm-thick poly(vinyliden fluoride–trifluoroethylene) [P(VDF–TrFE)] film. It has been found that, in this structure, the CNT transistor can operate as a memory transistor with an appropriate voltage margin.

Carbon nanotube (CNT) transistors demonstrate high mobility but also experience off-state leakage due to the small effective mass and band gap. The lower limit of off-current (IMIN) was measured in electrostatically doped CNT metal-oxide-semiconductor field-effect transistors (MOSFETs) across a range of band gaps (0.37 to 1.19 eV), supply voltages (0.5 to 0.7 V), and extension doping levels (0.2 to 0.8 carriers/nm). A nonequilibrium Green's function (NEGF) model confirms the dependence of IMIN on CNT band gap, supply voltage, and extension doping level. A leakage current design space across CNT band gap, supply voltage, and extension doping is projected based on the validated NEGF model for long-channel CNT MOSFETs to identify the appropriate device design choices. The optimal extension doping and CNT band gap design choice for a target off-current density are identified by including on-current projection in the leakage current design space. An extension doping level >0.5 carrier/nm is required for optimized on-current.

Variable behavior has been observed in several mechanisms found in biological neurons, resulting in changes in neural behavior that might be useful to capture in neuromorphic circuits. This paper presents a neuromorphic cortical neuron with synaptic neurotransmitter-release variability, which is designed to be used in neural networks as part of the Biomimetic Real-Time Cortex project. This neuron has been designed and simulated using carbon nanotube (CNT) transistors, which is one of several nanotechnologies under consideration to meet the challenges of scale presented by the cortex. Some research results suggest that some instances of variability are stochastic, while others indicate that some instances of variability are chaotic. In this paper, both possible sources of variability are considered by embedding either Gaussian noise or a chaotic signal into the neuromorphic or synaptic circuit and observing the simulation results. In order to embed chaotic behavior into the neuromorphic circuit, a chaotic signal generator circuit is presented, implemented with CNT transistors that could be embedded in the electronic neural circuit, and simulated using CNT SPICE models. The circuit uses a chaotic piecewise linear 1-D map implemented by switched-current circuits. The simulation results presented in this paper illustrate that neurotransmitter-release variability plays a beneficial role in the reliability of spike generation. In an examination of this reliability, the precision of spike timing in the CNT circuit simulations is found to be dependent on stimulus (postsynaptic potential) transients. Postsynaptic potentials with low neurotransmitter release variability or without neurotransmitter release variability produce imprecise spike trains, whereas postsynaptic potentials with high neurotransmitter-release variability produce spike trains with reproducible timing.

Novel two types of Carbon Nano Tube (CNT)-based multiplexers are introduced The first device is a solid-state transmission gate (t-gate) multiplexer that uses CNT as a channel in the Field Effect Transistors (FET) of both n-FET and p-FET types that are used. Because of its very small size, it has been shown that a CNT-based FET switches reliably using much less power than a silicon-based device, and thus the new device will consume less power than traditional t-gate multiplexer. The second new CNT device uses the fundamental Lorentz magnetic force from the basic laws of Electromagnetics as a switching mechanism between two conducting CNTs. The implementation of multiple-valued Galois logic using the new CNT devices is shown. Since a multiplexer is a fundamental logic block, the new devices can have a wide range of applications in a wide variety of nano circuits.

The electrical vapor sensor based on carbon nanotubes (CNTs) has attracted wide attention due to its excellent conductivity, stable interfacial structure, and low dimensional quantum effects. However, the conductivity and contact interface activity were still limited by the random distribution of coated CNTs, which led to limited performance. We developed a new strategy to unify the CNT directions with image fractal designing of the electrode system. In such a system, directional aligned CNTs were gained under a well-modulated electric field, leading to microscale CNT exciton highways and molecule-scale host-guest site activation. The carrier mobility of the aligned CNT device is 20-fold higher than that of the random network CNT device. With excellent electrical properties, such modulated CNT devices based on fractal electrodes behave as an ultrasensitive vapor sensor for methylphenethylamine, a mimic of illicit drug methamphetamine. The detection limit reached as low as 0.998 ppq, 6 orders of magnitude sensitive than the reported 5 ppb record based on interdigital electrodes with random distributed CNTs. Since the device is easily fabricated in wafer-level and compatible with the CMOS process, such a fractal design strategy for aligned CNT preparation will be widely applied in a variety of wafer-level electrical functional devices.

Several mechanisms have been proposed for the origin of unipolar p-type behavior in carbon nanotube (CNT) transistors including (i) O2 molecules trapped in the metal/CNT contact, (ii) OH radicals on the SiO2 surface, and (iii) O2/H2O coupling on the CNT surface. Nevertheless, the underlying mechanism is far from being clearly understood. Using density functional calculations, we propose that the SiO2 substrate induces wetting of moisture, which enhances the formation of CNT–OH complexes via several intermediate chemical reactions. As a consequence, the CNT–OH complex provides deep (occupied and unoccupied) acceptor levels near the valence band, acting as an electron trap center, which is the main cause of the asymmetric unipolar behavior of CNT transistors. Based on the theory, I–V characteristics are proposed in terms of gate bias and work function difference between metal and CNTs.

We have investigated microwave power detection based from carbon nanotube (CNT) thin films and chemical vapor deposition (CVD) grown graphene. Our experiments indicate that power detection from the CNT devices is primarily due to bolometric mechanisms. While, power detection from the graphene devices is primarily due to signal rectification. Both enabling materials are relatively inexpensive and easily blanketed on a variety of substrates{enabling low-cost/disposable, surface-conformal power sensors for wideband spectrum sensing applications. However, it is significantly less challenging to pattern and integrate CNT thin films than it is to do the same with graphene. CNT thin film and graphene power detectors were realized by fabricating metallic Corbino disc test structures over these enabling materials. Such test structures are convenient for on-wafer characterization with ground-signal probes. The CNT devices were also evaluated with transient current-versus- voltage traces and microwave reflection spectroscopy to respectively measure thermal time constants and values of complex conductivity. The bolometer performance of these devices was gauged in terms of power detection sensitivity, noise equivalent power, and dynamic range. The measurements were performed with 915 MHz test signals and yielded sensitivities as high as 0.36 mV/mW at room temperature and 2.3 mV/mW when cooled with liquid nitrogen. Similarly, graphene Corbino disc test structures were characterized with 433.92 MHz test signals and yielded power detection sensitivities of 3.25 mV/mW (at room temperature) and 5.43 mV/mW (at 80 K). These devices feature gate control over the channel conductance, which contributed a frequency-limiting parasitic capacitance. Our investigations revealed that rectification, due to characteristic nonlinear current versus voltage behavior, was more prevalent in the graphene than bolometric detection, due to Joule heating.

A random telegraph signal appears at a smaller absolute gate bias for a larger absolute drain-source bias in a carbon nanotube transistor. Its mechanism is attributed to a defect located in the drain side of the Schottky barrier carbon nanotube transistor with Ti∕Au as contact material. Furthermore, room temperature random telegraph signal is presented for both semiconducting and metallic carbon nanotubes, indicating the need to include random telegraph signal as a noise source for carbon nanotube transistors.

A random telegraph signal appears at a smaller absolute gate bias for a larger absolute drain-source bias in a carbon nanotube transistor. Its mechanism is attributed to a defect located in the drain side of the Schottky barrier carbon nanotube transistor with Ti/Au as contact material. Furthermore, room temperature random telegraph signal is presented for both semiconducting and metallic carbon nanotubes, indicating the need to include random telegraph signal as a noise source for carbon nanotube transistors. © 2006 American Institute of Physics. DOI: 10.1063/1.2402224

Molecularly imprinted polymers immobilized on carbon nanotube

Detection of low intensity light down to a few photons requires photodetectors with high gain. A new photodetector is reported based on C60 -sensitized aligned carbon nanotube (CNT) transistors with an extremely high responsivity of 108 A W-1 (gain > 108 ) in the ultraviolet and visible range, and 720 A W-1 (gain = 940) in the infrared range. In contrast to most sensitized phototransistors that operate on the photogating effect, the new photodetector operates on the modulation of the electrons scattering in the CNTs, leading to negative photoconductivity. Comparison with similar photodetectors using random CNT networks shows the benefit of using aligned CNTs. At room temperature, the aligned CNT photodetectors are demonstrated to detect a few tens of photons per CNT.

To circumvent the high carbon nanotube (CNT) growth temperature and poor adhesion with the substrates that currently plague CNT implementation, we proposed using CNT transfer technology enabled by open-ended CNTs. The process is featured with separation of CNT growth and CNT device assembly. Field emission testing of the as-assembled CNT devices is in good agreement with the Fowler-Nordheim (FN) equation, with a field enhancement factor of 4,540. This novel technique shows promising applications for positioning CNTs on temperature-sensitive substrates and for the fabrication of field emitters, electrical interconnects, and thermal management structures in microelectronics packaging.

Carbon nanotube (CNT) has been considered as an ideal channel material for building highly sensitive gas sensors. However, the reported H2 sensors based on CNT always suffered from the low sensitivity or low production. We developed the technology to massively fabricate ultra-highly sensitive H2 sensors based on solution derived CNT network through comprehensive optimization of the CNT material, device structure, and fabrication process. In the H2 sensors, high semiconducting purity solution-derived CNT film sorted by poly[9-(1-octylonoyl)-9 H-carbazole-2,7-diyl](PCz) is used as the main channel, which is decorated with Pd nanoparticles as functionalization for capturing H2. Meanwhile, Ti contacts are used to form a Schottky barrier for enhancing transferred charge-induced resistance change, and then a response of resistance change by 3 orders of magnitude is achieved at room temperature under the concentration of ∼311 ppm with a very fast response time of approximately 7 s and a detection limit of 890 ppb, which is the highest response to date for CNT H2 sensors and the very first time to show the sub-ppm detection for H2 at room temperature. Furthermore, the detection limit concentration can be improved to 89 ppb at 100 °C. The batch fabrication of CNT film H2 sensors with ultra-high sensitivity and high uniformity is ready to promote CNT devices to application for the first time in some specialized field.

ABSTRACTPhotoinduced conductivity changes and effects of thermal annealing in carbon nanotube transistors have been examined. Low-intensity ultraviolet light significantly reduces the p-channel conductance while simultaneously increasing the n-channel conductance. A combination of optical absorption and electron transport measurements reveals that these changes occur without variations in dopant concentrations. Measurements with different metals reveal that UV induces oxygen desorption from the electrodes rather than from nanotubes. In Ti-nanotube contact where the Schottky barrier plays an important role, photodesorption of oxygen mainly occurs from the native oxide of Ti electrodes. Decrease in the p-channel conductance arises from the metal work function change which causes larger hole Schottky barrier. Non-Schottky Pd-contacted nanotube transistors do not show photodesorption effects with low intensity UV. Thermal annealing of nanotube transistors with Ti/Au electrodes also leads to the disappearance of the photodesorption effects. However, a noticeable p-doping is observed to upon air exposure after thermal annealing.

A type of carbon nanotube transistors, which would be suitable for large-scale integration, has been fabricated from vertically aligned carbon nanotubes. We fabricated highly ordered carbon nanotubes, which are selectively grown on the patterned aluminum oxide nanotemplates. Each device element is formed on a vertical carbon nanotube attached to a bottom (source) and upper (drain) electrodes and a gate electrode, which is electrostatically switchable. The transistors can be integrated in large arrays with the potential for tera-level density (2×1011/cm2). The vertical carbon nanotube transistor shows ON/OFF switching operation at 30 K.