The present invention relates generally to the use of nanotubes in electrical circuits and, in particular, to circuits for voltage and current reference supplies.
Carbon nanotubes (CNTs) are allotropes of carbon that take the form of cylindrical carbon molecules. First observed in the 1950's, CNT's have novel properties that make them potentially useful in a wide variety of applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized. The diameter of a single-walled nanotube (SWNT) is typically 1 to 30 nm, and its length can be up to orders of micrometers. The band gap for conduction electrons and therefore the electrical conductivity of a carbon nanotube can be adjusted by means of its tube parameters, such as, for example, its diameter and its chirality.
Nanotubes can be produced not only from carbon but also from other elements such as boron nitride.
Single-walled nanotubes (SWNT) are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. Single-walled nanotubes are the most likely candidate for miniaturizing electronics past the micro electromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors. One useful application of SWNTs is in the development of intramolecular field effect transistors (FETs). The production of the first intra-molecular logic gate using SWNT FETs has recently become possible and CNT FET's have been proposed for multi-level logic circuits. In particular, the geometry dependent threshold voltage of a CNT FET has been used to design a family of ternary logic devices.
The accompanying figures, in which like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to the use of nanotubes in band gap circuits. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
Carbon nanotubes have many properties, from their unique dimensions to an unusual current conduction mechanism, that make them ideal components of electrical circuits. It is known that as the nanotube diameter increases, more wavevectors are allowed in the circumferential direction. Since the band gap in semiconducting nanotubes is inversely proportional to the tube diameter, the band gap (the energy difference between the top of the valence band and the bottom of the conduction band, which is the energy that an electron must reach in order to flow free in a semiconductor) approaches zero at large diameters. For example, at a nanotube diameter of about 3 nm, the band gap becomes comparable to thermal energies at room temperature.
The tube diameter d is related to the chirality vector (n, m) by
d=√{square root over (n2+m2+nm)}×0.0783 nanometers.
The band gap Eg is related to the diameter d (in nanometers) by
where is the reduced Planck constant,
=h/2π, and vF is the Fermi velocity.
Different band gaps can be engineered from the same basic material (carbon) by manipulating the diameter of the nanotube during manufacture.
In accordance with some embodiments of the present invention, carbon nanotubes having different band gaps are combined to create analog or digital circuits.
The principle of band-gap silicon circuits is well known. For example, a silicon band gap voltage reference circuit relies on two groups of transistors running at different emitter current densities. The rich transistor will typically run at a multiple (10 times, for example) of the density of the lean ones, and will cause a voltage difference between the base-emitter voltages of the two groups. This difference voltage is usually amplified and added to a collector/emitter voltage. The total of these two voltages adds up to voltage that is approximately the band gap of silicon.
The silicon has a single band gap determined by its molecular structure. In contrast, the band gap of a carbon nanotube is dependent on its diameter. In a CNT FET, the band gap is related to the threshold voltage VTH, since the energy bands move up, or down, due to application of a gate control voltage VG.
The current mirror circuit 102 comprises three P-channel transistors, 208, 210 and 212. The gates of the transistors are coupled and a voltage VDD is applied to the circuit. The transistors 208, 210 and 212 have nominally the same characteristics and form a current mirror that balances the currents in current outputs 110, 112 and 114. In
A steady state condition exists in which current flows through all of the transistors in the current mirror circuit 102. This steady state may be attained by use of a start-up circuit (106 in
Referring again to
The transistors 202 and 204 may be carbon nanotube field effect transistors (CNT FET's). In the embodiment shown in
Since current output 110 is set to be equal to current output 112 by the current mirror circuit 102 and the diameter of the first nanotube is smaller than the diameter of the second nanotube, the current density in the second transistor 204 is lower than that in the first transistor 202. The voltage across resistive element 206 is proportional to the difference in threshold voltages, which is, in turn, dependent upon the difference in band gap voltages. This voltage develops a current across the resistive element 206. This current is the same current flowing through transistor 204 and the output current 112. The output current 114 is set to be equal to current output 112 by the current mirror circuit 102. Therefore, the output current 114 is also proportional to the difference in threshold voltages, which is, in turn, dependent upon the difference in band gap voltages of the first and second nanotubes.
The corresponding transistor 404 in the voltage circuit 108 may also be replaced by a plurality of transistors (404, 404′. etc.) arranged in parallel. The number of transistors, and their nanotube diameters, may be chosen to match transistors 204, 204′ etc.
The use of nanotube transistors in voltage and current reference circuits provides a circuit designer with additional parameters, thereby increasing the flexibility in the circuit design. In contrast to silicon transistors, which have a single band gap of 1.205V, carbon nanotube transistors have band gaps that depend upon their diameter. The nanotube diameter may be varied to achieve a desired reference voltage.
The circuit topology and the nanotube diameters are output, in printed or electronic format for example, as at least part of the circuit design.
In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.