matlab cntfet simulation code
#1

HELLO! I am Tapender Singh, trying to model the CNTFET but there are some problem in my modelling work. Please provide the code if you can.
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#2

Carbon nanotubes are promising candidates for passive or active elements in post-CMOS nanoelectronics. Avouris brought experimental evidences that CNTFETs could have better performances than ultimate silicon MOSFETs [1]. Simi- lar results were obtained in the group of H. Dai [2]. For now, many experimental groups are studying CNT based devices, covering various topics:
- the influence of the metal used for the contacts to con-
trol charge injection efficiency,
- their frequency dependent behaviours,
- Their new promising functionalities.
However, to predict the ultimate performances of these novel nanodevices, and to further offer guidance and cost reduction of the technological development, accurate and reliable simu- lation tools appears as key issues. Indeed, as the micro- electronics companies show an increasing concern with post- CMOS technologies, there is a strong demand for simulation tools. In particular, companies that develop circuit simulation softwares try to include modelling of nanodevices-based cir- cuits in their available products. For CNT devices, as well as for other molecular electronics devices, it is necessary to de- velop new models, since the standard approximations and models used for MOS electronics may lose their applicability range. A first difficulty arises from the fact that intrinsic transport properties of CNTs are much less understood than their bulk semi-conductors counterparts, widely used in mi- croelectronics. The energy band structure strongly depends on the nanotube helicity and radius, and the scattering mecha- nisms (phonons, impurities) have been theoretically evaluated in a reduced number of cases.
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• Y.Maheswar is currently pursuing Ph.D in ECE JNT University, INDIA, Ph: 9642359378.

E.Mail: yarasimaheswar[at]gmail.com

• Dr.B.LRaju is currently working as principal in AURO- RA Engg college, INDIA, Ph: 9440925929

E.Mail: blraju2[at]gmail.com

• Dr.K.Soundara rajan is currently working as professor in

ECE JNT University, INDIA, Ph: 8897511366

E.Mail: soundararajan_jntucea[at]yahoo.com

A second serious difficulty is the experimentally observed dispersion of device characteristics. This dispersion is general- ly assigned to the variability of metal/nanotube interfaces at the molecular scale. So, to obtain predictive circuit simulation results, it is mandatory to precisely understand transport phe- nomena in CNTFET at the molecular scale. Hence, considering a single walled, semiconducting carbon nanotube as the chan- nel of a CNTFET including source, drain, gate electrodes, the circuit compatible model should describe the transistor one- dimensional (1-D) electrostatics in its ballistic limit of perfor- mance. Moreover, for convenient and efficient circuit simula- tion, the circuit-compatible model has to be suitable for a wide range of CNTFETs diameters ranging from 0.6 to 3 nm and for varied chiralities as long as they are semiconducting. Alt- hough such model derives from required approximations and simplifications to develop analytical expressions, strong foun- dation of the underlying physics of operation [3] determine the accuracy of the circuit transfer (dc) characteristics together with the transient response. There are various reports in the literature discussion about applications of CNTFET, however many of the papers report the CNT characterization. In this work, we discuss the physical structure of CNT, its equivalent circuit, model files and also present an experimental setup to simulate CNTFETs. We also discuss the electrical and VI char- acteristics of CNTFET.
Section II describes the carbon nanotube structure and its physical properties. Section III presents the CNT FET model and its equivalent circuit and Section IV presents the simula- tion results of CNTFET model using Hspice as well as Matlab. Section V discusses the conclusion.

2 CARBON NANOTUBE

The discovery of carbon nanotubes actually originates from fullerences, a hollow spherical structure of an allotrope of carbon C-60, which was discovered in 1985. The spherical structure is also called buckminster fullerence, or buckyball, named after a noted architect Richard Buckmister. Some of the scientists who discovered this, including Harold Kroto, Robert

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Curl and Richard Smalley were awarded the 1996 Nobel Prize in Chemistry [1]. Figure 1 shows the three dimensional view of a C-60 buckyball [2].

CNT’s Unique Structure

Carbon nanotubes, an extended structure of a fullerence in length, are composed of ultrathin carbon fiber with nanome- ter-size diameter and micrometer-size length. Figure 2 illus- trates what they look like. Basically, they are sheets of graphite rolled up into a tube. What is so unique about CNT is due to the different electrical and thermal conductivities they exhibit when their hexagonal structures are orientated differently. For instance, armchair, chiral and zig-zag structure allow the CNT to act like metal, semiconductor and insulator, respectively. Therefore, such characteristics have been employed in semi- conductor industries. The difference in orientation can be de- termined by a method of measurement termed chiral vector. Like many other scientific discoveries, CNT was accidentally discovered as a by-product by a Japanese scientist, Sumio Iiji- ma, in the carbon cathode used for the arc-discharging process preparing fullerences [3]. Such process later became one of the techniques to synthesize CNT. In next section, some methods of synthesizing CNT will be discussed. The applications of CNT are not limited to semiconductor industry. Over the years, other applications of CNT have been proposed in areas that range from chemistry, physics, and materials to biology. Synthesis and Purification
CNT can be classified into two types: multi-walled CNT (MWCNT) and single-walled CNT (SWCNT). MWCNT was discovered earlier than the latter, which is comprised of 2 to 30 concentric graphitic layers, whose diameter ranges from 10 to
50 nm and more than 10 um in length. SWCNT, on the other
hand, is a lot thinner due to its single graphite layer and has diameter from 1.0 to 1.4 nm.
Systhesis
The synthesis technique of CNT has been studied ex-
tensively and now is being prepared by many methods: arc-
discharging, laser ablation and catalytic decomposition of hy-
drocarbons. Other methods such as electrolysis and solar en- ergy have been proposed. More effort has been put into the study of the synthesis of SWCNT. However, both MWCNT and SWCNT share some techniques in terms of synthesis. They both require some metals as catalyst, including Fe, Co, Ni. SWCNT requires alloys, such as Fe/Co, Fe/Ni, Ni/Co, Ni/Cu, etc. The following are some of the techniques to syn- thesize CNTs.

Electric arc Discharge

Two carbon electrodes are kept with a gap in between. When high current, about 80 A is passed through the electrodes where gap is filled with helium under 300 torr. Cylindrical deposit then grows at about 2 to 3 mm per minute. This cath- ode deposit contains two portions: the inside is a black fragile core and the outside a hard shell. Generally, two parameters determine the quantity and the quality of the CNT deposit. Laser Ablation
MWCNT synthesized using laser ablation results much short-
er in length compared with arc discharge. Surprisingly, the same technique is able to produce SWCNT in excellent high
yield. Therefore, great effort has been put into synthesizing

SWCNT using this method.
Fig. 1. A 3-D view of a bucky ball Carbon-60

Fig. 2. Three different orientations of carbon nanotubes in chi- ral vectors

Catalytic Decomposition of Hydrocarbon

High quality of MWCNT can be mass produced with this method with lower cost. Metals such as Fe and Ni act as cata- lysts to break down the hydrocarbons passing through quartz tube in gas phase with a heating source. The straightness of the MWCNT produced using this method, however, is not as satisfying as that using arc-discharge. And thus the electrical conductivity is affected.

Purification

The synthesized CNTs require isolation processes since some nanoparticles, basically the by-products of carbon not in de- sired structure, as well as oxidized carbon, are produced in the synthesis process. Centrifugation, micro-filtration and chro- matography are employed to separate the CNTs of different lengths.

3 DESCRIPTION OF CNTFET

A. Carbon nanotube within a transistor configuration

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For now, the transistor configurations based on single walled carbon nanotubes (SWNTs) are various: the Schottky barrier CNTFET (SB-CNTFET) [4], the conventional CNTFET (C- CNTFET) featuring a doping profile similar to n-MOSFET [5], the dual-gate CNTFET exhibits n- or p-type unipolar behavior tunable by electrostatic doping [6] and the tunneling CNTFET [7]. The subject of this paper is the conventional CNTFET. Its structure albeit simple provides a behavior like normal MOSFET with yet ballistic transport [8]. A typical layout of a MOSFET-like CNFET device is shown in Fig. 1. The CNT channel region is undoped, whereas the other regions are heavily doped, acting as both the source/drain extension re- gion and/or interconnects between two adjacent devices (un- contacted source–gate/gate–drain configurations). This sec- tion describes the modeling of one single intrinsic channel of CNFET, as shown in Fig. 1 (inset), which is a starting point toward the complete device model reported in [1]. For MOSFET-like CNFET, since positive-FET (pFET) behavior is similar to negative FET (nFET), we only describe the equations for nFET in this paper, although we implemented both nFET and pFET for the SPICE simulations.

III. Model of the Intrinsic Channel Region


This part models the intrinsic channel region of CNFET with a near-ballistic transport and without any parasitic capacitance and parasitic resistance. The equivalent circuit model is shown as Fig. 2. Fig. 2(a) is the equivalent circuit implemented with HSPICE, and Fig. 2(b) and © is the other two possible imple- mentations for the transcapacitance network. The Fermi-level profiles and the energy-band diagram in the channel region with a ballistic transport are shown in

Fig. 3(a). The potential differences μs − μ_s and μd − μ_d are determined by both the applied bias and the property of the source/drain extension regions. We will treat the nonballistic transport and the potential drop at the source/drain extension
region and the contacts in the complete device model [1].
Fig. 1. Three-dimensonal device structure of CNFETs with
multiple channels, high-k gate dielectric material, and related parasitic gate capacitances. In this example, three CNFETs are fabricated along one single CNT. The channel region of CNTs is undoped, whereas the other regions of CNTs are heavily doped. The inset shows the 3-D device structure of CNFET that is modeled in this work, with only the intrinsic channel region.

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Fig. 2. Equivalent circuit model for the intrinsic channel region of CNFET. (a) Nine-capacitor model, assuming that the carrier distribution along the channel is uniform. Exxx is the voltage- controlled voltage source, and the potential of Vxxx is equal to
the controlling voltage source. Rdummy is a largevalue (>1E15) resistor to keep the circuit stable. (b) Five-capacitor model and © six-capacitor model, assuming that all the car- riers from +k branches are assigned to the source and that all the carriers from −k branches are assigned to the drain. Building blocks of CNT model
Current sources

– Thermionic current contributed by the semiconducting subbands (Isemi)

– Current contributed by the metallic subbands (Imetal)

– Leakage current (Ibtbt) caused by the band-to-band tun-

neling (BTBT) mechanism through the semiconducting
subbands
The single-walled CNT (SWCNT), with chiralities (n1, n2), the
diameter (DCNT) is given by



Isemi current

The total current flowing from the drain to the source

– TLR and TRL are the transmission probability of the car-

riers at the substate

– where Vch,DS and Vch,GS denote the Fermi potential

differences near the source side within the channel

– Em,l is the carrier energy at the substate



Imetal current

For metallic subbands of metallic nanotubes, the cur-



rent includes both the electron and hole currents

Ibtbt current

In the subthreshold region, particularly with negative gate bias (nFET), the BTBT current from drain to source becomes significant. There are two possible tunneling regions: the “n”- shape region 1 and the “L”-shape region 2

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International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July-2013 1910

ISSN 2229-5518.
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