Abstract:
A carbon nanotube based memory device comprises a set of three concentric carbon nanotubes having different diameters. The diameters of the three concentric carbon nanotubes are selected such that an inner carbon nanotube is semiconducting, and intershell electron transport occurs between adjacent carbon nanotubes. Source and drain contacts are made to the inner carbon nanotube, and a gate contact is made to the outer carbon nanotube. The carbon nanotube based memory device is programmed by storing electrons or holes in the middle carbon nanotube through intershell electron transport. Changes in conductance of the inner carbon nanotube due to the charge in the middle shell are detected to determine the charge state of the middle carbon nanotube. Thus, the carbon nanotube based memory device stores information in the middle carbon nanotube in the form of electrical charge.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a carbon nanotube structure, and particularly to a multiwalled carbon nanotube memory device and methods of manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     As scaling for conventional CMOS integrated circuits approaches quantum mechanical limits, alternative nanostructures and materials have been investigated in the semiconductor industry. Of such nanostructures and materials, carbon nanotubes (CNTs) offer excellent intrinsic properties that are suitable for high performance nanoscale devices. 
     A key advantage of CNTs over conventional CMOS devices is that scaling limitations of MOSFETs due to boundary scattering of electrons from imperfect interfaces are solved naturally in CNTs which have a smooth, well coordinated graphene structure with no bonds to the outside. This enables CNTs to retain excellent transport properties to much smaller lateral dimensions than silicon. The small radius and possibility of completely surrounding the CNT by a gate provide excellent electrostatic confinement of channel electrons, enabling the channel length to be scaled down to very small dimensions, and their small size would enable high packing densities. Band structure calculations of CNTs according to P. Avouris and J. Chen, “Nanotube electronics and optoelectronics,” Materials Today, Vol. 9, pp. 46-54, (2006) show that conduction and valence bands are mirror images of each other, i.e., both electrons and holes should share equally good transport properties. This indicates suitability of CNTs for a general-purpose high-performance complementary circuit technology. 
     As is now well known, CNTs can be either metallic or semimetallic, depending on their chirality and have a bandgap which is inversely proportional to their diameter for the semiconducting tubes. A useful relation, derived from tight binding calculations, between the diameter and the band gap, E g  of a CNT is
 
 E   g =γ(2 d   C-C /√{square root over (3)} d   CNT ),  equation (1)
 
where γ is the hopping matrix element, d C-C  is the C—C bond distance, and d CNT  is the diameter of the carbon nanotube. Inclusion of electron-electron interactions raises the size of the bandgap, E g  significantly. For a 1 nm nanotube, the band gap is roughly 1 eV. The best transport measurements and device characteristics have been obtained on rather large diameter nanotubes, with their diameter in the 1.7-3 nm range. The idealized electron/hole dispersion relation is hyperbolic in shape, with a quasi parabolic “effective mass” regime at lower energies and a linear “constant velocity” regime at higher energies, where the limiting velocity, ν lim , is ˜5-10×10 7  cm/sec according to G. Pennington and N. Goldsman, “Semiclassical transport and phonon scattering of electrons in semiconducting carbon nanotubes,” Phys. Rev. B 68, 045426 (2003).
 
     P. Avouris et al, “Carbon Nanotube Electronics,” Proceedings of the IEEE, Vol. 91, No. 11, pp. 1772-1984, November (2003) discloses carbon nanotube field effect transistors, and compares the performance characteristics of the carbon nanotube field effect transistors with the performance characteristics of silicon based field effect transistors. Further, methods of forming carbon nanotube base integrated circuits are also disclosed. 
     One application of a carbon nanotube is formation of a switching device employing the carbon nanotube in a field effect transistor structure.  FIG. 1  shows a vertical cross-sectional view of an exemplary carbon nanotube field effect transistor according to P. Avouris et al. along a lengthwise direction of the carbon nanotube  130 . The exemplary carbon nanotube field effect transistor includes a back gate dielectric layer  120  comprising silicon oxide, a back gate electrode  112  formed in a semiconductor substrate  110 , and source and drain metal contacts  140 . Optionally, a top gate dielectric layer (not shown) and a top gate electrode (not shown) may be additionally formed to control conduction of current along the carbon nanotube. The exemplary semiconductor device functions as an electronic switch in the same manner as a semiconductor based field effect transistor. 
     Multiwalled carbon nanotubes, which contain multiple concentric carbon nanotubes having different diameters, are also known in the art.  FIG. 2  shows a model of a double wall carbon nanotube structure including an inner carbon nanotube and an outer carbon nanotube. When the distance between the inner nanotube and the outer nanotube is an atomic scale dimension, intershell electron transport occurs. Further, the intershell electron transport in multiwall carbon nanotubes having a wall-to-wall distance of about 0.3 nm have shown quantum conductance behavior. B. Bourlon et al., “Determination of the Intershell Conductance in Multiwalled Carbon Nanotubes,” Phys. Rev. Letters, Vol. 93, No. 17, pp. 176806-1˜176806-4, October (2004) disclose that the intershell transport is tunnel type and that measured intershell transmission is consistent with estimations based on the overlap between π-orbitals and neighboring shells. 
     Another application of a carbon nanotube is its use as a tip of a scanning electron microscope. P. Liu et al., “Peeling and Cutting a Multi-Walled Carbon Nanotube inside a Scanning Electron Microscope,” Proc. of the 2 nd  IEEE International Conference on Nano/Micro Engineered and Molecular Systems, pp. 286-289, January (2007) discloses a method of employing an electron beam to cut individual shells, or individual carbon nanotubes, to fabricate a desired structure from a multiwall carbon nanotube structure including multiple concentric carbon nanotubes.  FIG. 3  shows a tip for a scanning electron microscope that may be manufactured by the methods disclosed by P. Liu et al. 
     While some prototype semiconductor devices employing carbon nanotubes have been proposed as described above, the variety of available semiconductor devices based on carbon nanotubes is limited. Particularly, functional semiconductor circuitry requires other functional components including memory devices that can store information. 
     In view of the above, there exists a need for a carbon nanotube based memory device to enable storage of information in a carbon nanotube based semiconductor circuit, and methods of manufacturing the same. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the needs described above by providing a carbon nanotube device employing three concentric carbon nanotubes that store electronic charge in a middle carbon nanotube and methods of manufacturing the same. 
     In the present invention, a carbon nanotube based memory device comprises a set of three concentric carbon nanotubes having different diameters. The diameters of the three concentric carbon nanotubes are selected such that an inner carbon nanotube is semiconducting, and intershell electron transport occurs between a middle carbon nanotube and each of the inner carbon nanotube and an outer carbon nanotube under a suitable voltage bias between the inner carbon nanotube and the outer carbon nanotube. Source and drain contacts are made to the inner carbon nanotube, and a gate contact is made to the outer carbon nanotube. The carbon nanotube based memory device is programmed by storing electrons or holes in the middle carbon nanotube through intershell electron transport. Changes in conductance of the inner carbon nanotube due to the charge in the middle shell are detected to determine the charge state of the middle carbon nanotube. Thus, the carbon nanotube based memory device stores information in the middle carbon nanotube in the form of electrical charge. 
     According to an aspect of the present invention, a carbon nanotube structure is provided, which comprises: 
     a set of three concentric carbon nanotubes located on a substrate, the set including:
         an inner carbon nanotube having a first diameter;   a middle carbon nanotube having a second diameter, wherein the second diameter is greater than the first diameter; and   an outer carbon nanotube having a third diameter, wherein the third diameter is greater than the second diameter;       

     a conductive gate contact structure abutting the outer carbon nanotube; 
     a conductive source side contact structure abutting one side of the inner carbon nanotube; and 
     a conductive drain side contact structure abutting another side of the inner carbon nanotube. 
     In one embodiment, the inner carbon nanotube has a first length and the outer carbon nanotube has a second length, and the first length is greater than the second length. 
     In another embodiment, the middle carbon nanotube and the outer carbon nanotube have a same length and two pairs of vertically coincident edges. 
     In even another embodiment, each of a first distance between the inner carbon nanotube and the middle carbon nanotube and a second distance between the middle carbon nanotube and the outer nanotube is from about 0.2 nm to about 0.5 nm. 
     In yet another embodiment, a first charge tunneling rate between the inner carbon nanotube and the middle carbon nanotube and a second charge tunneling rate between the middle carbon nanotube and the outer carbon nanotube are not equal. 
     In still another embodiment, the first diameter is from about 1.0 nm to about 40 nm. 
     In still yet another embodiment, the substrate further comprises a dielectric layer abutting the conductive gate structure. 
     In a further embodiment, the conductive source side contact structure and the conductive drain side contact structure abut the dielectric layer. 
     In an even further embodiment, the carbon nanotube structure further comprises a protective dielectric layer laterally abutting the conductive source side contact structure and the conductive drain side contact structure and abutting the outer carbon nanotube and the inner carbon nanotube. 
     In a yet further embodiment, the protective dielectric layer abuts a first pair of substantially circular end walls of the middle carbon nanotube and a second pair of substantially circular end walls of the outer carbon nanotube. 
     In a still further embodiment, the protective dielectric layer comprises one of a dielectric oxide, a dielectric nitride, and a dielectric polymer. 
     According to another aspect of the present invention, a method of operating a carbon nanotube structure is provided. The method comprises: 
     providing a carbon nanotube structure including:
         a set of three concentric carbon nanotubes located on a substrate, the set including:
           an inner carbon nanotube having a first diameter;   a middle carbon nanotube having a second diameter, wherein the second diameter is greater than the first diameter; and   an outer carbon nanotube having a third diameter, wherein the third diameter is greater than the second diameter;   
           a conductive gate contact structure abutting the outer carbon nanotube;   a conductive source side contact structure abutting one side of the inner carbon nanotube; and   a conductive drain side contact structure abutting another side of the inner carbon nanotube;       

     storing an electrical charge in the middle carbon nanotube; and 
     measuring electrical characteristics of the carbon nanotube structure, wherein the electrical characteristics of the carbon nanotube structure is affected by an amount of the electrical charge in the middle carbon nanotube. 
     In one embodiment, the electrical charge is stored in the middle carbon nanotube by applying a voltage bias across the inner carbon nanotube and the outer carbon nanotube. 
     In another embodiment, a first charge tunneling rate between the inner carbon nanotube and the middle carbon nanotube and a second charge tunneling rate between the middle carbon nanotube and the outer carbon nanotube are not equal. 
     In yet another embodiment, the electrical characteristics of the carbon nanotube structure is transconductance of the inner carbon nanotube. 
     In still another embodiment, the electrical characteristics of the carbon nanotube structure is a threshold voltage of the carbon nanotube structure. 
     According to yet another aspect of the present invention, a method of forming a carbon nanotube structure is provided, which comprises: 
     forming set of three concentric carbon nanotubes on a substrate, the set including:
         an inner carbon nanotube having a first diameter;   a middle carbon nanotube having a second diameter, wherein the second diameter is greater than the first diameter; and   an outer carbon nanotube having a third diameter, wherein the third diameter is greater than the second diameter;       

     forming a conductive gate contact structure directly on the outer carbon nanotube; 
     forming a conductive source side contact structure directly on one side of the inner carbon nanotube; and 
     forming a conductive drain side contact structure directly on another side of the inner carbon nanotube. 
     In one embodiment, the method further comprises cutting the outer carbon nanotube and the middle carbon nanotube, wherein the middle carbon nanotube and the outer carbon nanotube have a same length and two vertically coincident pairs of edges after the cutting, and wherein the inner carbon nanotube protrudes out from each of the two vertically coincident pairs of edges. 
     In another embodiment, the method further comprises forming a protective dielectric layer over the substrate and the set of three concentric carbon nanotubes. 
     In yet another embodiment, the method further comprises patterning the protective dielectric layer to expose two end portions of the inner carbon nanotube, wherein the conductive source side contact structure and the drain side contact structure are formed directly on the two end portions of the inner carbon nanotube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view of an exemplary prior art carbon nanotube transistor along a length of a carbon nanotube. 
         FIG. 2  is a view of a three dimensional model of a multiwall carbon nanotube structure having two shells, i.e., two nested carbon nanotubes. 
         FIG. 3  is a view of a model of a multiwall carbon nanotube having three shells, which is employed as a tip of a scanning electron microscopy tool. 
         FIG. 4  is a schematic view of a set of three concentric carbon nanotubes according to the present invention. 
         FIGS. 5-10  are sequential vertical cross-sectional views of an exemplary carbon nanotube structure according to the present invention in a vertical plane along a lengthwise direction of a set of three concentric carbon nanotubes. 
         FIG. 11  is a vertical cross-sectional view of the carbon nanotube structure of  FIG. 10  along the plane B-B′. The plane A-A′ corresponds to the plane of the vertical cross-sectional view of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As stated above, the present invention relates to a multiwall carbon nanotube memory device and methods of manufacturing the same, which are now described in detail with accompanying figures. It is noted that like and corresponding elements are referred to by like reference numerals. 
     Referring to  FIG. 4 , a set of three concentric carbon nanotubes is shown, which is employed to form a carbon nanotube structure according to the present invention. The set of three carbon nanotubes comprises an inner carbon nanotube  40  having a first diameter d 1 , a middle carbon nanotube  50  having a second diameter d 2 , and an outer carbon nanotube  60  having a third diameter d 3 . The first diameter d 1  is from about 1.0 nm to about 40 nm, and preferably from about 1.3 nm to about 10 nm, and more preferably from about 1.7 nm to about 4.0 nm. Preferably, the inner carbon nanotube  40  is semiconducting. 
     The second diameter d 2  is greater than the first diameter d 1 , and the third diameter d 3  is greater than the second diameter d 2 . The difference between the second diameter d 2  and the first diameter d 1  may be from about 0.4 nm to about 1.0 nm, i.e., the middle carbon nanotube  50  and the inner carbon nanotube  40  are separated by a distance from about 0.2 nm to about 0.5 nm. The difference between the third diameter d 3  and the second diameter d 2  may be from about 0.4 nm to about 1.0 nm, i.e., the outer carbon nanotube  60  and the middle carbon nanotube  50  are separated by a distance from about 0.2 nm to about 0.5 nm. Once formed, the set of three carbon nanotubes maintains concentricity among the three carbon nanotubes due to Van der Waals force between adjacent carbon nanotubes. 
     Preferably, the distance between the inner carbon nanotube  40  and the middle carbon nanotube  50  is tuned to provide electrical charge tunneling between them at a first tunneling tunneling rate. Likewise, the distance between the middle carbon nanotube  50  and the outer carbon nanotube is tuned to provide electrical charge tunneling between them at a second charge tunneling rate. Preferably, the first charge tunneling rate and the second charge tunneling rate are not equal for a given voltage bias between the middle carbon nanotube  50  and the inner carbon nanotube  40  or the outer carbon nanotube  60 . 
     Two end portions of the inner carbon nanotube  40  protrude out from the edges of the middle carbon nanotube  50  and the outer carbon nanotube  60 . A source side contact structure (not shown) is formed directly on a first protruding portion  42  of the inner carbon nanotube  40  on one side, and a drain side contact structure (not shown) is formed directly on a second protruding portion  44  of the inner carbon nanotube  40  on the other side. A gate contact structure (not shown) is formed directly on the outer carbon nanotube. 
     Charge is stored in the middle carbon nanotube  50  by applying an electrical bias between the inner carbon nanotube  40  and the outer carbon nanotube  60 . The asymmetry between the first tunneling rate and the second tunneling rate induces charge in the middle carbon nanotube. The waveform of the electrical bias may be a constant voltage pulse having an on state and an off state, or may have a time dependent waveform such as a sawtooth waveform. The amount of charge stored in the middle carbon nanotube stores information that is programmable and erasable. The information is “read” or detected by operating the set of three concentric carbon nanotube as a semiconductor device. Specifically, by applying an electrical bias between the first protruding portion  42  and the second protruding portion  44  and another electrical bias to the outer carbon nanotube  60 , the transconductance of the inner carbon nanotube  40  or the threshold voltage, i.e., the voltage bias needed on the outer carbon nanotube  60  to induce a predefined significant level of current, is measured. The value of the measured parameters indicates the amount of charge in the middle carbon nanotube  50 . The information stored in the middle carbon nanotube may be binary or may be a base greater than 2, i.e., may be ternary, quaternary, etc. 
     Referring to  FIG. 5 , a vertical cross-sectional view of an exemplary carbon nanotube structure according to the present invention at an initial manufacturing step is shown. The exemplary carbon nanotube structure comprises a conductive gate contact structure  30  formed on a substrate  8 . The substrate  8  comprises a substrate layer  10  and a dielectric layer  20 . The substrate layer  10  may comprise a conductor, a semiconductor, or an insulator. The dielectric layer  20  comprises a dielectric material such as a dielectric oxide, a dielectric nitride, or a dielectric polymer. For example, the dielectric material may comprise silicon oxide. 
     The conductive gate contact structure  30  comprises a conductive material such as a metal, a conductive metallic alloy, or a doped semiconductor material. The metal may be an elemental metal or an alloy of multiple elemental metals. Exemplary conductive metallic alloys include, but are not limited to, TaN, TiN, AlTaN, WN, and other refractory metal nitrides. The doped semiconductor material includes doped silicon, doped germanium, and doped semiconductor alloys. The conductive gate contact structure  30  may be formed by blanket deposition of a conductive layer on the dielectric layer  20 , followed by patterning of the conductive layer. The patterning may employ lithographic methods and an etch, such as an anisotropic dry etch or a wet etch. The thickness of the conductive gate contact structure  30  may be from about 3 nm to about 300 nm, and typically from about 10 nm to about 100 nm, although lesser and greater thicknesses are explicitly contemplated herein. 
     Referring to  FIG. 6 , a set of three concentric carbon nanotubes is thereafter placed on the conductive gate contact structure  30 . The set of three concentric carbon nanotubes comprises an inner carbon nanotube  40 , a middle carbon nanotube  50 , and an outer carbon nanotube  60 . The set of three concentric carbon nanotubes may be provided by methods known in the art. For example, methods of providing multiwall carbon nanotube structures as described in B. Bourlon et al. or P. Liu et al. (vide supra) may be utilized to provide the set of three concentric carbon nanotubes. The inner carbon nanotube  40  has a first diameter d 1 , the middle diameter  50  has a second diameter d 2 , and the outer carbon nanotube  60  has a third diameter d 3 , each of which satisfies the conditions of the first diameter d 1 , the second diameter d 2 , and the third diameter d 3  of the set of three concentric carbon nanotubes described in  FIG. 4 . 
     An inner-tube-enclosed space  35  is the cylindrical space enclosed by the inner carbon nanotube  40 . A first inter-carbon-nanotube space  45  is the annular and cylindrical space between the inner carbon nanotube  40  and the middle carbon nanotube  50 . A second inter-carbon-nanotube space  55  is the annular and cylindrical spacer between the middle carbon nanotube  50  and the outer carbon nanotube  60 . The inner-tube-enclosed space  35 , the first inter-carbon-nanotube space  45 , and the second inter-carbon-nanotube space  55  are concentric and have a cylindrical symmetry. 
     The inner carbon nanotube  40 , a middle carbon nanotube  50 , and an outer carbon nanotube  60  may, or may not, have a same length at this point. Preferably, the outer carbon nanotube  60  extends beyond a lateral dimension of the conductive gate contact structure  30 . 
     While formation of the conductive gate contact structure  30  precedes placement of the set of three concentric carbon nanotubes in the description of the present invention, formation and patterning of a conductive gate contact structure after the placement of the set of three concentric carbon nanotubes is also explicitly contemplated herein. 
     Referring to  FIG. 7 , the lengths of the outer carbon nanotube  60  and the middle carbon nanotube  50  are adjusted as necessary so that two end portions of the inner carbon nanotube  40  protrude outside the edges of the middle carbon nanotube  50  and the outer carbon nanotube  60 . The adjustment of the lengths of the middle carbon nanotube  50  and the outer carbon nanotube  60  may be effected by cutting end portions of each of the middle carbon nanotube  50  and the outer carbon nanotube  60  with a laser beam, an electron beam, or an ion beam. The end portions thus cut off are removed. 
     Preferably, the edges of the middle carbon nanotube  50  and the outer carbon nanotube  60  are made to be vertically coincident at both ends of the middle carbon nanotube  50  and the outer carbon nanotube  60 . Thus, the middle carbon nanotube  50  and the outer carbon nanotube  60  have a same length, which is herein referred to as a second length L 2 , and two pairs of vertically coincident edges, i.e., a first pair of vertically coincident edges on one side and a second pair of vertically coincident edges on the other side. The second length L 2  may be from about 5 nm to about 1,000 nm, and typically from about 20 nm to about 200 nm. 
     The inner carbon nanotube  40  protrudes out of each pair of vertically coincident edges. Thus, the length of the inner carbon nanotube  40 , which is herein referred to as a first length L 1 , is greater than the second length L 2 . 
     While the lengths of the middle carbon nanotube  50  and the outer carbon nanotube are adjusted after placement on the conductive gate contact structure  30  in the description of the present invention, adjustment of the lengths of the middle carbon nanotube  50  and the outer carbon nanotube prior to the placement of the set of the three carbon nanotubes ( 40 ,  50 ,  60 ) on the conductive gate contact structure is also explicitly contemplated herein. 
     Referring to  FIG. 8 , a protective dielectric layer  70  is formed over the set of the three concentric carbon nanotubes ( 40 ,  50 ,  60 ) and the dielectric layer  20 . The protective dielectric layer  70  may comprise one of a dielectric oxide, a dielectric nitride, and a dielectric polymer. Exemplary materials for the dielectric oxide include various silicon oxides including doped or undoped silicate glass formed by chemical vapor deposition (CVD) or spin-on coating and curing. Exemplary materials for the dielectric nitride include silicon nitride. Exemplary materials for the dielectric polymer include, but are not limited to, various polymethylmethacrylates (PMMAs), polyisoprenes, polybutadienes, polyvinylpyridines, polyethyleneoxides, polyethylenes, polyorganosilicates, polyferrocenyldimethylsilanes, etc. 
     The protective dielectric layer  70  has a thickness that exceeds the sum of the thickness of the conductive gate contact structure  30  and the third diameter d 3  (See  FIG. 6 ), which is the diameter of the outer carbon nanotube  60 . The protective dielectric layer  70  protects the set of the three concentric carbon nanotubes ( 40 ,  50 ,  60 ) from exposure to oxidants, moisture, and other contaminants as well as providing electrical isolation among conductive components to be subsequently formed. The protective dielectric layer  70  may, or may not, be planarized by any recess etch or other planarization methods. 
     Referring to  FIG. 9 , the protective dielectric layer  70  is patterned to expose both ends of the inner carbon nanotube  40 . Preferably, the outer carbon nanotube  60  and the middle carbon nanotube  50  are not exposed. The patterning of the protective dielectric layer  70  may be effected by lithographic methods employing a photoresist (not shown) followed by an etch, which may be a wet etch or an anisotropic dry etch such as a reactive ion etch. The etch is selective to the inner carbon nanotube  40 . Optionally, a portion of the conductive gate contact structure  30  may be exposed at this step. 
     Referring to  FIGS. 10 and 11 , vertical cross-sectional views of the exemplary semiconductor structure are shown such that  FIG. 10  shows a vertical cross-sectional view along the lengthwise direction the inner carbon nanotube  40  and including a diameter of the inner carbon nanotube  40 , and  FIG. 11  shows another vertical cross-sectional view perpendicular to the lengthwise direction of the inner carbon nanotube  40 . A conductive material is deposited on the inner carbon nanotube  40  and, if present, an exposed portion of the conductive gate contact structure  30 . A portion of the conductive material that is formed directly on a first end of the inner carbon nanotube  40  constitutes a conductive source side contact structure  82 . Another portion of the conductive material that is formed directly on a second end of the inner carbon nanotube  40  constitutes a conductive drain side contact structure  84 . If any exposed portion of the conductive gate contact structure  30  is present, yet another portion of the conductive material that is formed directly on the exposed portion of the conductive gate contact structure  30  constitutes a conductive gate contact structure extension  90 . 
     The exemplary carbon nanotube structure functions as a field effect transistor having a floating gate to store charges that affect the characteristics of the device. Particularly, the first end of the inner carbon nanotube  40  having the conductive source side contact structure  82  functions as a source of a transistor. The second end of the inner carbon nanotube  40  having the conductive drain side contact structure  84  functions as a drain of the transistor. The outer carbon nanotube  60  functions as a gate of the transistor. Thus, depending on the bias voltage across the conductive drain side contact structure  84  and the conductive source side contact structure  82  and the bias voltage between the outer carbon nanotube  60  and the conductive source side contact structure  82 , the transconductance and the current through the inner carbon nanotube  40  changes. 
     However, the exemplary carbon nanotube structure further comprises a “floating gate,” which comprises the middle carbon nanotube  50 . Due to the overlap of π-orbitals between adjacent carbon nanotubes, i.e., between the middle carbon nanotube  50  and each of the inner carbon nanotube  40  and the outer carbon nanotube  60 , charge tunneling occurs between adjacent carbon nanotubes when a voltage bias is applied across an adjacent pair of carbon nanotubes. Preferably, a first charge tunneling rate between the inner carbon nanotube  40  and the middle carbon nanotube  50  and a second charge tunneling rate between the middle carbon nanotube  50  and the outer carbon nanotube  60  are different under identical voltage bias. The various diameters (d 1 , d 2 , d 3 ) of the carbon nanotubes ( 40 ,  50 ,  60 ) may be tuned such that the ratio between the first charge tunneling rate and the second charge tunneling rate is between 0.0001 and 1 or between 1 and 10,000 under identical voltage bias conditions. The asymmetry between the first charge tunneling rate and the second charge tunneling rate is advantageously employed to store a charge in the middle carbon nanotube  50 . 
     To store electrical charges in the middle carbon nanotube  50 , a voltage bias is applied between the conductive gate contact structure  30  and the conductive source side contact structure  82 . The same voltage may be applied to the drain side contact structure  84 , or alternately, the drain side contact structure  84  may be left floating. The voltage bias between the outer carbon nanotube  60  and the inner carbon nanotube  40  induces a first tunneling current between the inner carbon nanotube  40  and the middle carbon nanotube  50  and a second tunneling current between the middle carbon nanotube  50  and the outer carbon nanotube  60 . A transient imbalance between the first tunneling current and the second tunneling current induces electrical charges in the middle carbon nanotube. The stored electrical charges may be positive or negative, i.e., holes may be stored or electrons may be stored in the middle carbon nanotube  50 . 
     The amount of charge, in addition to presence or absence of charge, in the middle carbon nanotube  50 , which functions as the floating gate, alters electrical characteristics of the transistor such as the transconductance and the current through the inner carbon nanotube  40 . Thus, by altering the amount of charge stores in the middle carbon nanotube  50 , information may be stored in the exemplary carbon nanotube structure, i.e., the exemplary carbon nanotube structure functions as a memory device. 
     The exemplary carbon nanotube structure may be employed to store a binary bit of information by operating the middle carbon nanotube  50  between two states, i.e., a charged state and an uncharged state. The charged state may store electrons or holes in the middle carbon nanotube. 
     Alternately, the exemplary carbon nanotube structure may be employed to store a ternary bit of information by operating the middle carbon nanotube  50  between three states, for example, a state charged with electrons, an uncharged state, and a state charged with holes, i.e., a state in which electrons are transferred out of the middle carbon nanotube  50 . 
     Yet alternately, the exemplary carbon nanotube structure may be employed to store information having a base greater than 3 by changing the amount of charge stored in the middle carbon nanotube  50 , and optionally, by changing the polarity of the charge stored in the middle carbon nanotube  50 . 
     While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations, which fall within the scope and spirit of the invention and the following claims.