Tunneling Carbon Nanotube Field Effect Transistors Research Articles

Improving the ON/OFF Current Ratio and Ambipolarity of Doping-Less Tunneling Carbon-Nanotube FET Using Drain Engineering Technique

This paper presents a novel structure utilizing a doping-less (DL) tunneling carbon nanotube field-effect transistor (T-CNTFET) with dual drain work functionality aimed at enhancing the ON/OFF current ratio. The proposed design features a zigzag carbon nanotube (CNT) of type (13, 0) with a diameter of 1 nm. It employs HfO2 as the gate oxide, which is 2 nm thick and has a dielectric constant of 16. The CNT serves as an intrinsic semiconductor for the source and drain regions, which are composed of metals with appropriate work functions. By selecting appropriate metals for the source and drain regions, the necessity for doping as a fabrication step is avoided, simplifying construction and reducing costs for the nanoscale device. This design includes two drain electrodes, each measuring 15 nm in length and having different work functions (DWFs). The work function of the drain positioned closest to the channel (DWFAC) is set at 3.9 eV, which is 0.5 eV lower than that of the CNT, while the other drain section has a work function of 3.4 eV, 1 eV lower than the CNT. The electrical properties of the device were examined through quantum numerical simulations using the non-equilibrium Green's function (NEGF) method. The results indicate that the proposed structure significantly decreases the OFF-state current and improves the ON/OFF current ratio. Additionally, the leakage current is substantially lowered, resulting in favorable changes to the device's ambipolarity, along with a reduction in hot carrier effects and subthreshold swing (SS).

New structure of tunneling carbon nanotube FET with electrical junction in part of drain region and step impurity distribution pattern

In this paper, by using electrical junction in part of drain region which includes stepwise doping distribution, a new structure is proposed for tunneling carbon nanotube field-effect transistors (T-CNTFETs). The modified device consists of two parts in the drain region, a region without electrical junction and a region having an electrical junction where the n-type impurity is entered stepwise along the drain region. Electronic features of the modified device are simulated by the simultaneous solution of both Schrodinger and Poisson equations in self-consistent routine. Simulation consequences demonstrate that the proposed modifications reduce OFF state current and enhance the ON/OFF ratio in comparison with the conventional structures. Moreover, delay time, the product of delay and power, and subthreshold slope as important features of the T-CNTFET are enhanced. Also, to more assessment of the suggested structure, variations of cut-off frequency parameters, including transconductance and gate capacitance have been investigated.

The use of a Gaussian doping distribution in the channel region to improve the performance of a tunneling carbon nanotube field-effect transistor

A new structure with a Gaussian doping distribution along the channel region is proposed to improve the performance of tunneling carbon nanotube field-effect transistors (T-CNTFETs). The new structure involves a Gaussian doping distribution in the channel region with a low level of doping at the sides that gradually increases towards the middle of the channel. The source doping is p-type, while the doping in the drain and channel regions is n-type. The doping distribution is uniform in the drain/source regions. To simulate the behavior of T-CNTFETs, the Poisson and Schrodinger equations are solved self-consistently using the nonequilibrium Green’s function formalism. The simulation results show that the proposed structure exhibits increased saturation current but decreased OFF-state current compared with the conventional structure (C-T-CNTFET), yielding a ~ 104 times higher current ratio for a gate length of 20 nm. The proposed structure also shows improvements in parameters such as the transconductance, gate capacitance, cutoff frequency, and delay compared with the conventional structure and can be considered to be a more appropriate option for different applications.

Nanoscale Sensor-Based Tunneling Carbon Nanotube Transistor for Toxic Gases Detection: A First-Principle Study

Carbon nanotube transistor is a spotlight components for toxic gas monitoring. In this regard, we present a new toxic gas molecules sensor base on a single-walled carbon nanotube transistor and use the Poisson and Schrodinger equations in a self-consistent manner and is modeled using non-equilibrium Green’s function. The reaction between gaseous molecules and single-walled carbon nanotubes can harness the charge and potential level inside the channel, which changes the electrical characteristics of the device. Investigations carried out over a 20-nm channel and at a temperature range between 200 and 400 K shows that the conductivity and current–voltage characteristic of the proposed Nano-sensor depend on temperature and gas conditions. According to the simulation results, the decrease in the temperature decreases the OFF-state current of the tunneling carbon nanotube field-effect transistor which leads to a decrease in the leakage current and the band-to-band tunneling. In addition, due to the temperature rise, the sensitivity is increased, and by increasing the dielectric constant of the gas, the on-state current of the device will also increase significantly. The maximum sensitivity for Carbonyl Chloride is 630 (nm/RIU).

A Computational Study of a Heterostructure Tunneling Carbon Nanotube Field-Effect Transistor

In this paper a heterostructure tunneling carbon nanotube field-effect transistor, namely H-T-CNTFET, is introduced. In the proposed structure, a carbon nanotube with a different chirality is used for the drain side compared to the source and the channel region. Characteristics of the proposed structure are numerically simulated by a mode-space non-equilibrium Green’s function formulism in the ballistic limit. Simulation results show that, compared to a conventional tunneling carbon nanotube field-effect transistor, the proposed structure has higher ON-current, better ambipolar behavior and better switching characteristics. In addition, the analog characteristics of the device, such as the transconductance (gm) and unity-gain frequency (fT), are also improved.

Cut Off Frequency Variation by Ambient Heating in Tunneling p-i-n CNTFETs

In this paper, we study the effects of ambient temperature variations on the high frequency behavior of tunneling carbon nanotube field effect transistors (T-CNTFETs). Device characteristics including transconductance, gate capacitance, and intrinsic cutoff frequency are extracted and their variations by temperature are investigated. To study and simulate a tunneling carbon nanotube field effect transistor we used self-consistent numerical solution of Poisson-Schrodinger equations and non-equilibrium Green's function method. The studies carried out on 20 nm channel length and in the range of 270 to 400°K. The results show that in T-CNTFETs by increasing the temperature, a simultaneous reduction in the transconductance (gm) and gate capacitance (Cg) is observed and there is a linear relation between them. The reduction in these two parameters causes to reduction in cutoff frequency which means the reduction in their ratio (gm/Cg) by temperature. By increasing the temperature from 270 to 400°K, at 20 nm channel length, about 33 GHz reduction in cutoff frequency is observed. Also, Simulation results show that the threshold voltage experiences slight increment, i.e. decrease in its ON-state current. In addition, the leakage current increases by increasing the ambient temperature.

Electrical Characteristics of T-CNTFET: Partially-Gated Channel vs Doping Engineering

The major limitation of tunneling carbon nanotube field-effect transistors (T-CNTFET) is the ambipolar behavior. In this paper, we show that this problem could be solved by using a partially-gated channel, or Gaussian doping method. The effect of using the two mentioned methods on the electrical characteristics of T-CNTFET has been examined. The non-equilibrium Green's functions (NEGF) in conjunction with Poisson's equation are solved self-consistently. The cutoff frequency (fT) and power-delay product (PDP) as well as the intrinsic delay (τ) have been considered as key parameters for frequency performance and speed characteristics. The simulation results show an improvement in the OFF-state current and the high-frequency performance as well as a suppression of the ambipolar conduction. These advantages of the proposed techniques make T-CNTFET suitable for low-power and high-speed applications.

Improving band-to-band tunneling in a tunneling carbon nanotube field effect transistor by multi-level development of impurities in the drain region

In this paper, in order to improve the performance of a tunneling carbon nanotube field effect transistor (T-CNTFET) a new structure is proposed using multi-level impurity distribution along the drain region. The new T-CNTFET structure consists of six parts in the drain with stepwise doping distribution. The impurities on the drain side are n -type and the length of each region is 5nm. Electronic features of the proposed structure are simulated by the solution of Poisson and Schrodinger equations and the self-consistent method using Non-equilibrium Green’s Function (NEGF). Simulation results show that the proposed structure reduces the band curvature near the drain-channel connection and widens the tunneling barrier. As a result, band-to-band tunneling and the OFF current are reduced and the ON/OFF current ratio increases in comparison with the conventional structure. In summary, by improving the subthreshold swing parameters, delay time, power delay product (PDP and cut-off frequency compared to the conventional structure, the proposed structure can be considered as a proper candidate for digital applications with high speed and low power dissipation.

Investigating the effect of some parameters of the channel on the characteristics of tunneling carbon nanotube field-effect transistor

This paper studies p-i-n tunneling carbon nanotube field-effect transistor to investigate the effect of various parameters of the channel on the characteristics of tunneling carbon nanotube field-effect transistor. Tunneling carbon nanotube field-effect transistor (T-CNTFET) has been simulated using non-equilibrium Green’s function (NEGF), and the transmission was conducted through inelastic scattering. Besides the evaluation of device performance, various parameters of the channel were also compared. One of the parameters is considered as the variable, while other parameters of the channel are constant. Then, improved characteristics were discussed by selection of some channel parameters. T-CNTFET with CNT (10, 0) with oxide thickness = 1 nm shows reduced sub-threshold swing (18 mV/decade).

THE EFFECT OF CARBON NANOTUBE CHIRALITY ON THE PERFORMANCE OF THE STRAINED TUNNELING CARBON NANOTUBE FETs

In this paper, using the non-equilibrium Green's function formalism (NEGF), the effect of the chirality of carbon nanotube (CNT) on the performance of the strained tunneling carbon nanotube field effect transistors (T-CNTFETs) has been investigated. In this work, all evaluations are done by this assumption that the ON current, I ON , is the main performance metric in the T-CNTFETs. The uniaxial strain has been considered in this work. On the other hand, for constructing a transistor with a desired I ON , a variety of CNTs with the appropriate uniaxial strain could be used. The results of doing some comparisons among these situations show that, the use of the small diameter CNTs with the appropriate uniaxial strain could lead to the transistors with the better switching behavior. Likewise, the use of the large diameter CNTs with the appropriate uniaxial strain could result to the transistors with a higher I ON /I OFF ratio.

Opitimization of tunneling carbon nanotube-FETs based on stair-case doping strategy

Due to the fact that either holes or electrons can band-to-band-tunnel (BTBT) through channelsource/drain contacts, tunneling carbon nanotube field effect transistors (T-CNFETs) suffer from ambipolar transporting characteristic. To suppress such ambipolar conductance, a novel device design based on stair-case doping strategy of drain region is proposed first in this paper. The dependences of available ON-OFF current ratio, practical ON-OFF current ratio with certain gate bias range and the average sub-threshold swing on the doping level of drain region are studied. Simulation results show that such device design can not only increase the ON-OFF current ratio largely but also result in a clear decrease of sub-threshold swing. At the same time, however, such stair-case doping strategy would broaden the depletion region and result in an increase of device area cost. Particular attention should be paid to the choice of doping level of drain region to make a proper tradeoff between power, speed and area in application.

Numerical study of the sub-threshold slope in T-CNFETs

The most attractive merit of tunneling carbon nanotube field effect transistors (T-CNFETs) is the ultra-small inverse sub-threshold slope. In order to obtain as small an average sub-threshold slope as possible, several effective approaches have been proposed based on a numerical insight into the working mechanism of T-CNFETs: tuning the doping level of source/drain leads, minimizing the quantum capacitance value via tuning the bias condition or increasing the insulator capacitance, and adopting a staircase doping strategy in the drain lead. Non-equilibrium Green's function based simulation results show that all these approaches can contribute to a smaller average inverse sub-threshold slope, which is quite desirable in high-frequency or low-power applications.

Performance optimization of carbon nanotube field effect transistors based on stair-case doping strategy

The ambipolar transporting characteristic is one of the most important factors that prevent the performance of Carbon Nanotube Field Effect Transistors (CNFETs) from further being improved. In order to reduce the ambipolar conductance and increase the ON-OFF current ratio of the device,a stair-case doping strategy in drain lead,which is suitable for not only the Conventional MOS-like CNFETs (C-CNFETs) but also Tunneling CNFET (T-CNFETs),is proposed in this paper. The non-equilibrium Greens function based simulation results show that this strategy can reduce the ambipolar conductance and increase the ON-OFF current ratio of the device effectively. Further study shows that many differences exist with using this stair-case doping strategy applied in C-CNFETs and T-CNFETs. First,the potential band pinning in stair-case doped C-CNFETs would weaken the ON-state performance of the device,while no band pinning exists in stair-case doped T-CNFETs. Second,applying such a stair-case doping strategy to both source and drain leads can further increase the device performances in C-CNFETs but not T-CNFEts. Third,the transporte property of T-CNFETs is dependent more strongly on the width of lightly doped drain region than that of C-CNFETs. However,certain device area would be costly because of using this stair-case doping strategy. So,much attention should be paid to the choice of device structure,doping concentration and lightly doped drain region width,to obtain a best tradeoff among speed,power and device area,in application.

Tunneling phenomena in carbon nanotube field‐effect transistors

AbstractIn the present article we will discuss the electronic trans‐ port properties of carbon nanotube field‐effect transistors (CNFETs). Three different device concepts will be studied in more detail: Schottky‐barrier CNFETs with metallic source and drain contacts, conventional‐type CNFETs with doped nanotube segments as source and drain electrodes and finally a new concept, the tunneling CNFET. As it turns out, tunneling phenomena play a prominent role in all three CNFET designs and determine their electrical behavior to a large extend. In addition, the one‐dimensionality of the electronic transport makes them ideally suited for novel device architecture such as the tunneling CNFET. Analytical as well as simulation results will be given and compared with each other and with experimental data in order to explain the different influences on the electronic transport in CNFETs and thus on the device behavior. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)