Nanotube Field-effect Transistors Research Articles

A method for creating reliable and low-resistance contacts between carbon nanotubes and microelectrodes

An ultrasonic bonding technique has been developed for bonding single wall carbon nanotubes (SWNTs) onto metal microelectrodes. The bonding was formed by pressing SWNTs against the electrodes with a vibrating press at an ultrasonic frequency. With this technology, low-resistance contacts are achieved between both metallic and semiconducting SWNTs and electrodes. After bonding, the effective Schottky barrier height between semiconducting SWNT and Ti electrode is as low as ∼6.6 meV in the ON-state and the barrier width is ∼0.9 nm at V g = 0. The performance of carbon nanotube field-effect transistors (FETs) fabricated by this ultrasonic bonding technique is also significantly improved, with a transconductance as high as 3.4 μS for solid-state back-gate individual nanotube FETs.

Ballisticity of nanotube field-effect transistors: Role of phonon energy and gate bias

We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube field-effect transistors (FETs). Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. For comparable electron-phonon coupling, a lower phonon energy leads to a larger degradation of drive current. Thus in semiconductor nanowire FETs, the drive current will be more sensitive than in carbon nanotube FETs because of the smaller phonon energies in semiconductors. Acoustic phonons and other elastic scattering mechanisms are most detrimental to nanotube FETs irrespective of biasing conditions.

One-dimensional screening effects in bulk-modulated carbon nanotube transistors

We report density-functional theory (DFT), atomistic simulations of the screening properties of carbon nanotube (CNT) field-effect transistors (FETs). Our attention has been focused on recently demonstrated [1, 2] bulk-modulated CNTFETs. Our calculations have been intended to explore, at an atomistic level, the effect of the one-dimensional screening properties on the physical mechanisms which govern the channel conductance modulation in these new devices. We find that in order to correctly describe charge response mechanisms in a small-diameter nanotube, a calculation including electron-electron interaction effects is necessary. Many-body effects tied to the attractive nature of the exchange interaction can produce an unconventional charge response which manifests itself in a negative quantum capacitance.

Relation between conduction property and work function of contact metal in carbonnanotube field-effect transistors

We have investigated the relation between the conduction property and the work functionof the contact metal in carbon nanotube field-effect transistors (NTFETs). The conductiontype and the drain current are dependent on the work function. In contrast to NTFETswith Ti and Pd contact electrodes, which showed p-type conduction behaviour, deviceswith Mg contact electrodes showed ambipolar characteristics and most of the devices withCa contact electrodes showed n-type conduction behaviour. This indicates that thebarrier height of the metal/nanotube contact is dependent on the work function ofthe contact metal, which suggests that the Fermi-level pinning is weak at theinterface, in contrast to conventional semiconductors such as Si and GaAs. Wehave also demonstrated nonlinear rectification current–voltage characteristics in ananotube quasi-pn diode with no impurity doping, in which different contactmetals with different work functions are used for the anode and the cathode.

Ultrasonic nanowelding of carbon nanotubes to metal electrodes

A simple ultrasonic nanowelding technique has been developed to reliably bond single-wallcarbon nanotubes (SWCNTs) onto metal electrodes, by pressing SWCNTs againstelectrodes under a vibrating force at ultrasonic frequency. The bonds formed have beendemonstrated to be mechanically robust. Using this technique, a stable low-Ohmic contactbetween SWCNTs and metal electrodes was achieved, with resistances in the range of8–24 kΩ fora 1 µm long metallic SWCNT at room temperature. The performance of carbonnanotube field-effect transistors (FETs) fabricated using this ultrasonicnanowelding method has also been greatly improved. Transconductance as high as3.6 µS among the solid-state back-gate individual nanotube FETs has been achieved.

Network Single-Walled Carbon Nanotube-Field Effect Transistors (SWNT-FETs) with Increased Schottky Contact Area for Highly Sensitive Biosensor Applications

Highly sensitive single-walled carbon nanotube-field effect transistor (SWNT-FET) devices, which detect protein adsorptions and specific protein-protein interactions at 1 pM concentrations, have been achieved. The detection limit has been improved 104-fold compared to the devices fabricated by photolithography. The substantially increased sensitivity is mainly due to the increased Schottky contact area which accommodates relatively more numbers of proteins even at very low concentration. The augmented number of proteins adsorbed on a device induces instant modulation of the work function of metal contact electrodes, which eventually changes the conductance of the device. Such devices have been attained by addressing metal electrodes on network-type SWNTs using a shadow mask on a tilted angle sample stage. The shadow mask allows metals to penetrate underneath the mask efficiently, therefore forming a thin and wide Schottky contact area on SWNT channels.

Extrapolated fmax for carbon nanotube field-effect transistors

Compact expressions are derived for the maximum operating frequency of carbon nanotubefield-effect transistors. The expressions are shown to be applicable over wide ranges ofphysical properties, parasitic resistances, and gate biases. The utility of the expressions isdemonstrated by their prompting of a conservative device design that should lead tofmax>0.5 THz.

Enhanced Channel Modulation in Dual-Gated Silicon Nanowire Transistors

Dual-gated silicon nanowire (SiNW) field-effect transistors (FETs) have been fabricated by using electron-beam lithography. SiNW devices (W approximately 60 nm) exhibit an on/off current ratio greater than 10(6), which is more than 3 orders of magnitude higher than that of control devices prepared simultaneously having a large channel width (approximately 5 microm). In addition, by changing the local energy-band profile of the SiNW channel, the top gate is found to suppress ambipolar conduction effectively, which is one of the factors limiting the use of nanotube or nanowire FETs for complimentary logic applications. Two-dimensional numerical simulations show that the gate-induced electrostatic control is improved as the channel width of the FETs decreases. Therefore, enhanced channel modulations can be achieved in these dual-gated SiNW devices.

Pressure-dependent Schottky barrier at the metal-nanotube contact

We carry out first-principles density-functional calculations to investigate the electronic structure of the gold-carbon nanotube contact. It is found that a pressure applied on the gold-nanotube contact shifts the Fermi level from the valence edge to the conduction edge of the carbon nanotube. This can explain the n-type transport behavior frequently observed in the nanotube field-effect transistor using the gold as electrodes. An atomistic model is proposed for a possible origin of the pressure when the nanotube is embedded in the gold electrode.

Photoresponse of Carbon Nanotube Field-Effect Transistors

Photoresponse of carbon nanotube field-effect transistors (FETs) is investigated using microscopic measurements. The nanotube FETs, with an isolated single-walled carbon nanotube (SWNT) for the channel, were fabricated by means of the position-controlled nanotube growth technique. An increase in the off-state current and the threshold-voltage shift of the FET were caused by laser illumination. The increase in the off-state current is attributed to photocurrent due to carriers excited in the SWNT channel. The excitation spectrum of the photocurrent had a peak corresponding to optical absorption by the third interband gap of the van Hove singularity of the semiconducting SWNT with a diameter of ∼2 nm. The photocurrent increased in proportion to incident laser power with a dynamic range over four orders of magnitude. The external quantum efficiency was 2×10-7. An inverter action to optical-signal input was observed near the threshold voltage of the FET. The responsivity was as high as 2×10-3 A/W for a single SWNT channel. This high responsivity is explained by the field-effect amplification phenomenon.

Modeling Hysteresis Phenomena in Nanotube Field-Effect Transistors

A model is developed to explain a hysteresis observed experimentally in nanotube field-effect transistors. The model explains the hysteresis through trapping of electrons in an oxide layer. The Fowler-Nordheim tunneling mechanism is held responsible for the electron injection. The influence of different parameters such as the sweeping rate or the range of the gate voltage on the hysteresis is studied and compared with experimental results.

Characteristics of a carbon nanotube field-effect transistor analyzed as a ballistic nanowire field-effect transistor

A general expression of the current–voltage characteristics of a ballistic nanowire field-effect transistor (FET) is derived. At T=0, the conductance, which is equal to the quantum conductance multiplied by the number of channels at zero bias, decreases stepwise toward current saturation as the drain bias is increased. The current–voltage characteristics of a single-wall carbon nanotube FET in ballistic conduction are discussed based on the band structure of the nanotube. When both the gate overdrive and the drain bias are equal to 1V, the device made of a (19,0) nanotube and a 2-nm high-k gate insulator (ε=40ε0) flows a current of 183μA, which amounts to a current density 48 times as large as the counterpart of a silicon device. The high performance originates from a high carrier density due to the enhanced gate capacitance, and a large carrier velocity caused by the large group velocity of the original graphene band. Quantum capacitance also plays an important role in the device’s characteristics.

Nanoelectronic Carbon Dioxide Sensors

Carbon nanotube field-effect transistors (NTFETs) coated with poly(ethyleneimine) (PEI) and starch polymers exhibit electrical conductance changes upon exposure to CO2 gas in air at ambient temperature (see Figure). This observation has furnished nanoelectronic CO2 sensors. Their small size and low power consumption has enormous potential in wireless sensing for industrial and medical CO2 sensor units.

A double-walled carbon nanotube field-effect transistor using the inner shell as its gate

Based on the low inter-shell conductance, we propose a design of the carbon nanotube (NT) field-effect transistor (FET), made of a hybrid double-walled carbon nanotube (DWNT) with the semiconducting outer shell as the conductance channel and the inner shell as the gate. Its performance at room temperature is analyzed according to the ballistic transport theory. Because the separation between the two shells of the DWNT is only 0.34 nm , this NT FET performs better than the present NT FETs and the ballistic MOSFETs because of its better gate control.

Chirality assignment of individual single-walled carbon nanotubes in carbon nanotube field-effect transistors by micro-photocurrent spectroscopy

We have proposed a possibility of chirality assignment of individual single-walled carbon nanotubes in nanotube field-effect transistors (FETs) by micro-photocurrent spectroscopy. The nanotube FETs were fabricated by utilizing position-controlled nanotube growth technique using alcohol chemical vapor deposition. Photocurrent signal that originated from a single single-walled carbon nanotube was obtained by using microscopic optical measurement system. A peak was observed at 1.73 eV in the photocurrent excitation spectrum. This corresponds to optical absorption by van Hove singularity of the nanotube with a diameter of ∼2 nm. Chirality of the nanotube in the nanotube FET was assigned to be (18, 10) by comparing the experimental results with theoretical calculation.

High performance field-effect transistors made of a multiwall CNx/C nanotube intramolecular junction

Field-effect transistors (FETs) based on an individual CNx/C nanotube (NT) were fabricated by focus ion-beam technology. The nanotube transistors exhibited n-type semiconductor characteristics, and the conductance of nanotube FETs can be modulated more than four orders of magnitude at room temperature. The electron mobility of a CNx/C NT FET estimated from its transconductance was as high as 3.84×103 cm2/V s. The n-type gate modulation could be explained as due the effect of bending of the valence band in the Schottky-barrier junction.

Interaction of Aromatic Compounds with Carbon Nanotubes: Correlation to the Hammett Parameter of the Substituent and Measured Carbon Nanotube FET Response

We have used field-effect transistor (FET) devices with semiconducting single-walled carbon nanotubes (SWNTs) as the conducting channels to study interactions of aromatic compounds with SWNTs. Electronic detection occurs through charge-transfer effects, monitored as the change of the gate voltage (Vg) dependence of the source-drain current Isd. For monosubstituted benzene compounds, we find that the shift of the Isd − Vg characteristic is proportional to the Hammett sigma values (σp) of their substituents.

Electrostatics of coaxial schottky-barrier nanotube field-effect transistors

Analytical and numerical methods are used to solve Poisson's equation for carbon nanotube field-effect transistors (FETs) with a cylindrical surrounding gate and Schottky-barrier contacts to the source and drain. The effect on the nanotube potential profile of varying the work functions of all the electrodes, and the thickness and permittivity of the gate dielectric, is investigated. From these results, the general trends to be expected in the above-threshold drain current-voltage characteristics of Schottky-barrier nanotube FETs are predicted. The unusual possibility of simultaneous electron and hole contributions to the drain current is revealed. The subthreshold characteristics are computed from a solution to Laplace's equation, and the subthreshold slope is found to depend on gate dielectric thickness in a different manner from that in other FETs.

Bipolar conduction and drain-induced barrier thinning in carbon nanotube FETs

The drain current-voltage (I-V) characteristics of Schottky-barrier carbon nanotube field-effect transistors (FETs) are computed via a self-consistent solution to the two-dimensional potential profile, the electron and hole charges in the nanotube, and the electron and hole currents. These out-of-equilibrium results are obtained by allowing splitting of both the electron and hole quasi-Fermi levels to occur at the source and drain contacts to the tube, respectively. The interesting phenomena of bipolar conduction in a FET, and of drain-induced barrier thinning (DIBT) are observed. These phenomena are shown to add a breakdown-like feature to the drain I-V characteristic. It is also shown that a more traditional, saturating-type characteristic can be obtained by workfunction engineering of the source and drain contacts.

Photoconductivity of Single Carbon Nanotubes

We observe infrared laser excited photoconductivity from a single carbon nanotube incorporated as the channel of an ambipolar field-effect transistor (FET). Electron−hole pairs are generated within the nanotube molecule, and the carriers are separated by an applied electric field between the source and drain contacts. The photocurrent shows resonances whose energies are in agreement with the energies of exciton states of semiconducting nanotubes of the appropriate diameter. The photocurrent is maximized for photons polarized along the direction of the carbon nanotube. Thus, the nanotube FET acts as a polarized photodetector with a diameter 1000 times smaller than the wavelength of the light it detects and has an estimated quantum efficiency of >10%. A photovoltage is observed when an asymmetric band lineup due to two nonequivalent Schottky barriers or an asymmetric coupling of the gate to the nanotube is present.