Synaptic Variability in a Cortical Neuromorphic Circuit

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.

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

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.

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.

Among the challenges hindering the integration of carbon nanotube (CNT) transistors in digital technology are the lack of a scalable self-aligned gate and complementary n- and p-type devices. We report CNT transistors with self-aligned gates scaled down to 20 nm in the ideal gate-all-around geometry. Uniformity of the gate wrapping the nanotube channels is confirmed, and the process is shown not to damage the CNTs. Further, both n- and p-type transistors were realized by using the appropriate gate dielectric-HfO2 yielded n-type and Al2O3 yielded p-type-with quantum simulations used to explore the impact of important device parameters on performance. These discoveries not only provide a promising platform for further research into gate-all-around CNT devices but also demonstrate that scalable digital switches with realistic technological potential can be achieved with carbon nanotubes.

Molecularly imprinted polymers immobilized on carbon nanotube

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.

Carbon nanotubes (CNTs) exhibit several technologically important characteristics such that metallic nanotubes can carry extremely large current densities; semiconducting nanotubes can be electrically switched on and off as field effect transistors (FETs), and so on CNT FETs with characteristics comparable to or exceeding state of the art Si based transistors have been demonstrated using a conventional FET design with high-κ 1) and SiO 2 dielectric 2) . In addition, CNTs have been successfully demonstrated as biological sensors with high sensitivity. It has been reported that the real time detection of single viruses 3) , small molecules 4) , and proteins 5) , 6) becomes is possible with biosensors that use CNT transistors as the active transducer. For these applications of CNTs, the control of the number of CNT between electrodes is quite important technology. However, it is quite difficult and has not been realized yet. It is therefore, indispensable to control it for the future applications of CNT. In the present study, we have established the new technology to control the number of the CNT one by one during the growth of CNT by monitoring the electrical current between electrodes, which is named as Digital Growth Process.

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.

Carbon nanotubes (CNTs) have been studied in recent years due to their exceptional electronic, opto-electronic, and mechanical properties. To explore the physics of carbon nanotube field-effect transistors (CNT-FETs) self-consistent quantum mechanical simulations have been performed. The performance of carbon nanotube-based transistors is analyzed numerically, employing the non-equilibrium Green?s function formalism. Electron-phonon interaction parameters, such as electron-phonon coupling strength and phonon energy, strongly depend on the chirality and the diameter of the carbon nanotube. The response of carbon nanotube based transistors is studied taking rigorously into account the effect of electron-phonon interaction.

Electrical biosensors, including transistor-based devices (i.e., BioFETs), have the potential to offer versatile biomarker detection in a simple, low-cost, scalable, and point-of-care manner. Semiconducting carbon nanotubes (CNTs) are among the most explored nanomaterial candidates for BioFETs due to their high electrical sensitivity and compatibility with diverse fabrication approaches. However, when operating in solutions at biologically relevant ionic strengths, CNT-based BioFETs suffer from debilitating levels of signal drift and charge screening, which are often unaccounted for or sidestepped (but not addressed) by testing in diluted solutions. In this work, we present an ultrasensitive CNT-based BioFET called the D4-TFT, an immunoassay with an electrical readout, which overcomes charge screening and drift-related limitations of BioFETs. In high ionic strength solution (1X PBS), the D4-TFT repeatedly and stably detects subfemtomolar biomarker concentrations in a point-of-care form factor by increasing the sensing distance in solution (Debye length) and mitigating signal drift effects. Debye length screening and biofouling effects are overcome using a poly(ethylene glycol)-like polymer brush interface (POEGMA) above the device into which antibodies are printed. Simultaneous testing of a control device having no antibodies printed over the CNT channel confirms successful detection of the target biomarker via an on-current shift caused by antibody sandwich formation. Drift in the target signal is mitigated by a combination of: (1) maximizing sensitivity by appropriate passivation alongside the polymer brush coating; (2) using a stable electrical testing configuration; and (3) enforcing a rigorous testing methodology that relies on infrequent DC sweeps rather than static or AC measurements. These improvements are realized in a relatively simple device using printed CNTs and antibodies for a low-cost, versatile platform for the ongoing pursuit of point-of-care BioFETs.

This work investigates the p-doping effect of Nitric Oxides (NOX) annealing on transistors with a channel from a network of carbon nanotubes (CNTs), with a focus on extension doping of top-gated transistors and RF characterization. NOX annealing at 100 °C for 1 h induces oxidation in CNTs, which introduces a doping band inside the nanotube band gap near the valence band. The benefits and drawbacks of NOX annealing on the performance of back-gated (BG) CNT transistors are characterized. The doping band is beneficial in that it reduces the Schottky barrier for hole injection and increases the hole carrier concentration in the channel. However, we also note that NOX annealing deteriorates the inverse subthreshold swing (SS) and drain-induced barrier lowering (DIBL) by introducing interface traps and worsens any existing short channel effects. Our capacitance analysis confirms that the increase in source-drain and interface-trap capacitances degrades both SS and short channel effects. To circumvent the drawbacks of NOX annealing, we employ it to create extension doping regions in p-type, top-gated transistors with a scaled high-k gate oxide, leading to an increase in drain current from 5.09 to 13.95 μA at VOV = -2 V and VDS = -1 V. Because the effect of NOX annealing is limited to the extension regions and has little impact on the channel region, negative effects on the SS and DIBL are mitigated. Finally, an RF transistor is fabricated, and NOX annealing is used to enhance the cutoff frequency by approximately 10-fold.

With this work we introduce a novel memristor in a lateral geometry whose resistive switching behaviour unifies the capabilities of bipolar switching with decelerated diffusive switching showing a biologically plausible short-term memory. A new fabrication route is presented for achieving lateral nano-scaled distances by depositing a sparse network of carbon nanotubes (CNTs) via spin-coating of a custom-made CNT dispersion. Electrochemical metallization-type (ECM) resistive switching is obtained by implanting AgAu nanoparticles with a Haberland-type gas aggregation cluster source into the nanogaps between the CNTs and shows a hybrid behaviour of both diffusive and bipolar switching. The resistance state resets to a high resistive state (HRS) either if the voltage is removed with a retention time in the second- to sub-minute scale (diffusive) or by applying a reverse voltage (bipolar). Furthermore, the retention time is positively correlated to the duration of the Set voltage pulse. The potential for low-voltage operation makes this approach a promising candidate for short-term memory applications in neuromorphic circuits. In addition, the lateral fabrication approach opens the pathway towards integrating sensor-functionality and offers a general starting point for the scalable fabrication of nanoscaled devices.