Effect of diameter variation in a large set of carbon nanotube transistors

A study involving a large number of carbon nanotube transistors reveals that the nanotube diameter and the metal contact material play key roles in determining the on- and off-state currents of these devices. The results are discussed in terms of the Schottky barrier at the metal-semiconductor junction and the variation of this barrier relative to the alignment of energy levels between the carbon nanotube and the metal.     

Carbon nanotube electronics

Presents experimental results on single-wall carbon nanotube field-effect transistors (CNFETs) operating at gate and drain voltages below 1V. Taking into account the extremely small diameter of the semiconducting tubes used as active components, electrical characteristics are comparable with state-of-the-art metal oxide semiconductor field-effect transistors (MOSFETs). While output as well as subthreshold characteristics resemble those of conventional MOSFETs, we find that CNFET operation is actually controlled by Schottky barriers (SBs) in the source and drain region instead of by the nanotube itself. Due to the small size of the contact region between the electrode and the nanotube, these barriers can be extremely thin, enabling good performance of SB-CNFETs.     

Novel Local Silicon-Gate Carbon Nanotube Transistors Combining Silicon-on-Insulator Technology for Integration

By taking advantage of the silicon-on-insulator technology and the in situ carbon nanotube (CNT) growth, new local silicon-gate carbon nanotube FETs (CNFETs) have been implemented in this paper. We propose an approach to integrate the CNFET onto the silicon CMOS platform for the first time. Individual device operation, batch fabrication, low parasitic capacitance, and better compatibility to the CMOS process were realized. The characteristics of the CNFETs are comparable to the state-of-the-art devices reported. The scaling effect, ambipolar conductance, Schottky barrier effect, and I–V characteristics noise were analyzed. The physical properties of the CNTs were also characterized.      

Circuit-Level Performance Benchmarking and Scalability Analysis of Carbon Nanotube Transistor Circuits

Carbon nanotubes (CNTs) show great promise as extensions to silicon CMOS due to their excellent electronic properties and extremely small size. Using a Carbon Nanotube Field Effect Transistor (CNFET) SPICE model, we evaluate circuit-level performance of CNFET technology in the presence of CNT fabrication-related nonidealities and imperfections, and parasitic resistances and capacitances extracted from the CNFET circuit layout. We use Monte Carlo simulations using the CNFET SPICE model to investigate the effects of three major CNT process-related imperfections on circuit-level performance: 1) doping variations in the CNFET source and drain regions; 2) CNT diameter variations; and 3) variations caused by the removal of metallic CNTs. The simulation results indicate that metallic CNT removal has the most impact on CNFET variation; less than 8% of CNTs grown should be metallic to reduce circuit performance variation. This paper also presents an analytical model for the scalability of CNFET technology. High CNT density (250 CNTs/mum) is critical to ensure that performance (delay and energy) gains over silicon CMOS are maintained or improved with shrinking lithographic dimensions.     

Frequency response of top-gated carbon nanotube field-effect transistors

The ac performance of carbon nanotube field-effect transistors (CNFETs) has been characterized using two approaches involving: 1) time- and 2) frequency-domain measurements. A high input impedance measurement system was used to demonstrate time-domain switching of CNFETs at frequencies up to 100 kHz. The low level of signal crosstalk in CNFETs fabricated on quartz substrates enabled frequency-domain measurements of the ac response of CNFETs in the megahertz range, over five orders of magnitude higher in frequency than previously reported ac measurements of CNFET devices.     

Imaging of the Schottky barriers and charge depletion in carbon nanotube transistors

The photovoltage produced by local illumination at the Schottky contacts of carbon nanotube field-effect transistors varies substantially with gate voltage. This is particularly pronounced in ambipolar nanotube transistors where the photovoltage switches sign as the device changes from p-type to n-type. The detailed transition through the insulating state can be recorded by mapping the open-circuit photovoltage as a function of excitation position. These photovoltage images show that the band-bending length can grow to many microns when the device is depleted. In our palladium-contacted devices, the Schottky barrier for electrons is much higher than that for holes, explaining the higher p-type current in the transistor. The depletion width is 1.5 mum near the n-type threshold and smaller than our resolution of 400 nm near the p-type threshold. Internal photoemission from the metal contact to the carbon nanotube and thermally assisted tunneling through the Schottky barrier are observed in addition to the photocurrent that is generated inside the carbon nanotube.     

Measuring Frequency Response of a Single-Walled Carbon Nanotube Common-Source Amplifier

Frequency response function (FRF) showing ac gain from a single-walled carbon nanotube transistor is presented. A top-gated carbon nanotube FET (CNFET) is configured as a common-source amplifier and the FRF of the amplifier is measured. Evidence of unambiguous signal amplification is observed in time domain as well as frequency domain up to a unity voltage gain frequency of approximately 560 kHz. The observed roll-off in frequency is solely due to the RC time constant of the measurement apparatus. A specifically designed circuit-compatible SPICE model for the CNFET is used to model both dc and ac characteristic with the same set of physical parameters. Good agreement between measurement and simulation is obtained. For a device without the parasitic load capacitance, we predict an intrinsic unity voltage gain frequency of 29 GHz and a cutoff frequency of $sim {50}$ GHz.      

Role of Doping in Carbon Nanotube Transistors With Source/Drain Underlaps

The effects of doping on the performance of coaxially gated carbon nanotube (CNT) field-effect transistors for both zero Schottky-barrier (SB) and doped carbon nanotube contacts are theoretically investigated. For ultrascaled CNTFETs in which the source/drain metal contacts lie 50 nm apart, there is no MOSFET-like contact CNTFET (C-CNTFET) with an acceptable on/off current ratio using a CNT of diameter ges1.5 nm and a source/drain voltage ges0.4 V. For CNTFETs with source/drain metal contacts either 50 nm or 100 nm apart, there is an optimal doping concentration of 10^-3 dopants per atom. The maximum on/off current ratios for the 50 nm CNT/5 nm gate and the 100 nm CNT/10 nm gate SB-CNTFETs are 5 times 10⁴ and 6 times 10⁵, respectively. Performance metrics of delay time, cutoff frequency, and LC frequency are presented and compared.     

Gas fingerprinting using carbon nanotubes transistor arrays

This paper deals with the fabrication of carbon nanotube field effect transistors (CNTFETs) for gas sensing applications. Such devices exploit the extremely sensitive change of the Schottky barrier heights between carbon nanotubes (CNTs) and drain/source metal electrodes: the gas adsorption creates an interfacial dipole that modifies the metal work function and so the band bending and the height of the Schottky barrier at the contacts. Our aim is to achieve the fingerprinting of a specific gas using a CNTFET based sensor array. This fingerprinting concept is based on the fact that the change of the metal electrode work function strictly depends on the metal/gas interaction. Consequently the CNTFET transfer characteristics will change specifically as a function of this interaction. To demonstrate this new concept, we have fabricated arrays of CNTFETs with different metal contacts: Au, Pd, Ti and Pt. Using these transistors, we have shown that a particular gas, in our case DiMethyl-Methyl-Phosphonate (DMMP, a sarin simulant), interacts specifically with each metal: 1 ppm of DMMP (15 min of exposure) reduces the transistor ON current by about 20% for Pt contacted CNTFETs and by nearly one order of magnitude for Pd contacted CNTFETs. We believe that this new approach can be applied for highly selective sensing of various gases, using ultra-compact, room temperature and very low power devices.     

Network single-walled carbon nanotube biosensors for fast and highly sensitive detection of proteins

Detection of proteins is powerfully assayed in the diagnosis of diseases. A strategy for the development of an ultrahigh sensitivity biosensor based on a network single-walled carbon nanotube (SWNT) field-effect transistor (FET) has been demonstrated. Metallic SWNTs (m-SWNTs) in the network nanotube FET were selectively removed or cut via a carefully controlled procedure of electrical break-down (BD), and left non-conducting m-SWNTs which magnified the Schottky barrier (SB) area. This nanotube FET exhibited ultrahigh sensitivity and fast response to biomolecules. The lowest detection limit of 0.5?pM was achieved by exploiting streptavidin (SA) or a biotin/SA pair as the research model, and BD-treated nanotube biosensors had a 2 × 10(4)-fold lower minimum detectable concentration than the device without BD treatment. The response time is in the range of 0.3-3?min.     

Quantitative Experimental Analysis of Schottky Barriers and Poole–Frenkel Emission in Carbon Nanotube Devices

In this paper, we investigated carbon nanotube FETs (CNT FETs) utilizing semiconducting single-walled CNTs (SWCNTs). Multiple devices, each of different metal source and drain contacts, were fabricated on a single SWCNT. Over specific temperature regimes, transport properties of the devices were found to be consistent with the Bethe theory of thermionic emission for Schottky contacts, and the Poole–Frenkel emission was dependent on the device position. As was expected, transport from thermionic emission over the barrier was found to be the dominant mechanism. Barriers of 25–41 meV were present, as found by activation energy analysis for temperatures ranging from 20 to 300 K for the devices. A Schottky diode was also fabricated on a separate nanotube using an ohmic contact at the Pd source and a Schottky contact for the Ag drain electrode. Assuming the same physical assumptions for an Si semiconductor device, the results indicate an ideality factor greater than 2, Schottky barrier of $sim$0.37 eV, and image charge lowering of $sim$0.1 eV.      

Study of gaseous interactions in carbon nanotube field-effect transistors through selective Si(3)N(4) passivation

Carbon nanotube field-effect transistors with Si(3)N(4) passivated source and drain contacts and exposed carbon nanotube channel show n-type characteristics in air. In contrast, by passivating only the source contact, a diode-like behavior with a maximum current rectification ratio of 4.6 × 10(3) is observed. The rectifying characteristic vanishes in a vacuum but recovers once the devices are exposed to air. From our experiments, key parameters, such as critical gas pressure, adsorption energy of oxygen molecules and the contact barrier height modulation, can be obtained for studying the gaseous interaction in the carbon nanotube devices.     

Properties of short channel ballistic carbon nanotube transistors with ohmic contacts

We present self-consistent, non-equilibrium Green's function calculations of the characteristics of short channel carbon nanotube transistors, focusing on the regime of ballistic transport with ohmic contacts. We first establish that the band line-up at the contacts is renormalized by charge transfer, leading to Schottky contacts for small diameter nanotubes and ohmic contacts for large diameter nanotubes, in agreement with recent experiments. For short channel ohmic contact devices, source-drain tunnelling and drain-induced barrier lowering significantly impact the current-voltage characteristics. Furthermore, the ON state conductance shows a temperature dependence, even in the absence of phonon scattering or Schottky barriers. This last result also agrees with recently reported experimental measurements.     

Contact Phenomena and Mott Transition in Carbon Nanotubes

We describe the transfer of electric charge in junctionsbetween a metal and carbon nanotube as well as betweenmetallic and semiconducting carbon nanotubes. The long rangeCoulomb interaction drastically modifies the charge transferphenomena in one-dimensional nanotube systems compared toconventional semiconductor heterostructures. Being broughtinto a contact with a metal, conducting nanotube accumulateselectric charge whose density decays slowly with the distancefrom the junction. The length of the Schottky barrier innanotube heterojunctions varies from the distances of theorder of the nanotube radius (nanometers) to the distances ofthe order of the nanotube length (microns) depending on adoping strength. The Schottky barrier height shows pronouncedasymmetry under the forward and reverse bias. This results inrectifying behavior of heterojunctions, in agreement withrecent experimental observations by Z. Yao et al. andP. McEuen et al. Finally, we discuss observability of recentlypredicted Mott insulating phase in metallic carbon nanotubes.     

Linear increases in carbon nanotube density through multiple transfer technique

We present a technique to increase carbon nanotube (CNT) density beyond the as-grown CNT density. We perform multiple transfers, whereby we transfer CNTs from several growth wafers onto the same target surface, thereby linearly increasing CNT density on the target substrate. This process, called transfer of nanotubes through multiple sacrificial layers, is highly scalable, and we demonstrate linear CNT density scaling up to 5 transfers. We also demonstrate that this linear CNT density increase results in an ideal linear increase in drain-source currents of carbon nanotube field effect transistors (CNFETs). Experimental results demonstrate that CNT density can be improved from 2 to 8 CNTs/μm, accompanied by an increase in drain-source CNFET current from 4.3 to 17.4 μA/μm.     

Self-assembly of carbon-nanotube-based single-electron memories

We demonstrate the wafer-scale integration of single-electron memories based on carbon nanotube field-effect transistors (CNFETs) using a process based entirely on self assembly. First, a "dry" self-assembly step based on chemical vapor deposition (CVD) allows the growth and connection of CNFETs. Next, a "wet" self-assembly step is used to attach a single 30-nm-diameter gold bead in the nanotube vicinity via chemical functionalization. The bead is used as the memory storage node while the CNFET operating in the subthreshold regime acts as an electrometer exhibiting exponential gain. Below 60 K, the transfer characteristics of gold-CNFETs show highly reproducible hysteretic steps. Evaluation of the capacitance confirms that these current steps originate from the controlled storage of single electrons with a retention time that exceeds 550 s at 4 K.     

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.     

Quantum mechanical simulation of hole transport in p-type Si Schottky barrier MOSFETs

A full quantum-mechanical simulation of p-type nanowire Schottky barrier metal oxide silicon field effect transistors (SB-MOSFETs) is performed by solving the three-dimensional Schr?dinger and Poisson's equations self-consistently. The non-equilibrium Green's function (NEGF) approach is adopted to treat hole transport, especially quantum tunneling through SB. In this work, p-type nanowire SB-MOSFETs are simulated based on the 3-band k.p method, using the k.p parameters that were tuned by benchmarking against the tight-binding method with sp3s* orbitals. The device shows a strong dependence on the transport direction, due to the orientation-sensitive tunneling effective mass and the confinement energy. With regard to the subthreshold slope, the [110] and [111] oriented devices with long channel show better performance, but they are more vulnerable to the short channel effects than the [100] oriented device. The threshold voltage also shows a greater variation in the [110] and [111] oriented devices with the decrease of the channel length.     

Carbon nanotube Schottky diode: an atomic perspective

The electron transport properties of semiconducting carbon nanotube (SCNT) Schottky diodes are investigated with atomic models using density functional theory and the non-equilibrium Green's function method. We model the SCNT Schottky diode as a SCNT embedded in the metal electrode, which resembles the experimental set-up. Our study reveals that the rectification behaviour of the diode is mainly due to the asymmetric electron transmission function distribution in the conduction and valence bands and can be improved by changing metal-SCNT contact geometries. The threshold voltage of the diode depends on the electron Schottky barrier height which can be tuned by altering the diameter of the SCNT. Contrary to the traditional perception, the metal-SCNT contact region exhibits better conductivity than the other parts of the diode.     

Supported lipid bilayer/carbon nanotube hybrids

Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube--with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes.