Abstract:
Nanotube-based logic circuitry is disclosed. Tri-stating elements add an enable/disable function to the circuitry. The tri-stating elements may be provided by nanotube-based switching devices. In the disabled state, the outputs present a high impedance, i.e., are tri-stated, which state allows interconnection to a common bus or other shared communication lines. In embodiments wherein the components are non-volatile, the inverter state and the control state are maintained in the absence of power. Such an inverter may be used in conjunction with and in the absence of diodes, resistors and transistors or as part of or as a replacement to CMOS, biCMOS, bipolar and other transistor level technologies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims the benefit under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/656,253, now U.S. Pat. No. 7,288,961, entitled Tri-State Circuit Using Nanotube Switching Elements and filed on Jan. 22, 2007, which is incorporated herein by reference in its entirety, which is a continuation of and claims the benefit under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/032,823, now U.S. Pat. No. 7,167,026, entitled Tri-State Circuit Using Nanotube Switching Elements and filed on Jan. 10, 2005, which is incorporated herein by reference in its entirety, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/581,071, Nonvolatile Carbon Nanotube Logic (NLOGIC) Tri-state Circuit filed on Jun. 18, 2004, which is incorporated herein by reference in its entirety. 
         [0002]    This application is related to the following references:
       U.S. patent application Ser. No. 10/917,794, now U.S. Pat. No. 7,115,960, filed on Aug. 13, 2004, entitled Nanotube-Based Switching Elements;   U.S. patent application Ser. No. 10/918,085, now U.S. Pat. No. 6,990,009, filed on Aug. 13, 2004, entitled Nanotube-Based Switching Elements With Multiple Controls;   U.S. patent application Ser. No. 10/918,181, now U.S. Pat. No. 7,071,023, filed on Aug. 13, 2004, entitled Nanotube Device Structure And Methods Of Fabrication;   U.S. patent application Ser. No. 10/917,893, now U.S. Pat. No. 7,138,832, filed on Aug. 13, 2004, entitled Nanotube-Based Switching Elements And Logic Circuits;   U.S. patent application Ser. No. 10/917,606, now U.S. Pat. No. 7,289,357, filed on Aug. 13, 2004, entitled Isolation Structure For Deflectable Nanotube Elements;   U.S. patent application Ser. No. 10/917,932, filed on Aug. 13, 2004, entitled Circuits Made From Nanotube-Based Switching Elements With Multiple Controls;   U.S. patent application Ser. No. 11/033,087, filed on Jan. 10, 2005, entitled Nanotube-Based Transfer Devices and Related Circuits;   U.S. patent application Ser. No. 11/033,089, now U.S. Pat. No. 7,288,970, filed on Jan. 10, 2005, entitled Integrated Nanotube and Field Effect Switching Device;   U.S. patent application Ser. No. 11/033,213, filed on Jan. 10, 2005, entitled Receiver Circuit Using Nanotube-Based Switches and Transistors;   U.S. patent application Ser. No. 11/033,215, filed on Jan. 10, 2005, entitled Receiver Circuit Using Nanotube-based Switches and Logic;   U.S. patent application Ser. No. 11/032,216, now U.S. Pat. No. 7,164,744, filed on Jan. 10, 2005, entitled Nanotube-based Logic Driver Circuits; and   U.S. patent application Ser. No. 11/032,983, now U.S. Pat. No. 7,161,403, filed on Jan. 10, 2005, entitled Storage Elements Using Nanotube Switching Elements.       
 
     
    
     BACKGROUND 
       [0015]    1. Technical Field 
         [0016]    The present application relates in general to logic circuits and in particular to logic circuits constructed using nanotube switching elements. 
         [0017]    2. Discussion of Related Art 
         [0018]    Digital logic circuits are used in personal computers, portable electronic devices such as personal organizers and calculators, electronic entertainment devices, and in control circuits for appliances, telephone switching systems, automobiles, aircraft and other items of manufacture. Early digital logic was constructed out of discrete switching elements composed of individual bipolar transistors. With the invention of the bipolar integrated circuit, large numbers of individual switching elements could be combined on a single silicon substrate to create complete digital logic circuits such as inverters, NAND gates, NOR gates, flip-flops, adders, etc. However, the density of bipolar digital integrated circuits is limited by their high power consumption and the ability of packaging technology to dissipate the heat produced while the circuits are operating. The availability of metal oxide semiconductor (“MOS”) integrated circuits using field effect transistor (“FET”) switching elements significantly reduces the power consumption of digital logic and enables the construction of the high density, complex digital circuits used in current technology. The density and operating speed of MOS digital circuits are still limited by the need to dissipate the heat produced when the device is operating. 
         [0019]    Digital logic integrated circuits constructed from bipolar or MOS devices do not function correctly under conditions of high heat or heavy radiation. Current digital integrated circuits are normally designed to operate at temperatures less than 100 degrees centigrade and few operate at temperatures over 200 degrees centigrade. In conventional integrated circuits, the leakage current of the individual switching elements in the “off” state increases rapidly with temperature. As leakage current increases, the operating temperature of the device rises, the power consumed by the circuit increases, and the difficulty of discriminating the off state from the on state reduces circuit reliability. Conventional digital logic circuits also short internally when subjected to heavy radiation because the radiation generates electrical currents inside the semiconductor material. It is possible to manufacture integrated circuits with special devices and isolation techniques so that they remain operational when exposed to heavy radiation, but the high cost of these devices limits their availability and practicality. In addition, radiation hardened digital circuits exhibit timing differences from their normal counterparts, requiring additional design verification to add radiation protection to an existing design. 
         [0020]    Integrated circuits constructed from either bipolar or FET switching elements are volatile. They only maintain their internal logical state while power is applied to the device. When power is removed, the internal state is lost unless some type of non-volatile memory circuit, such as EEPROM (electrically erasable programmable read-only memory), is added internal or external to the device to maintain the logical state. Even if non-volatile memory is utilized to maintain the logical state, additional circuitry is necessary to transfer the digital logic state to the memory before power is lost, and to restore the state of the individual logic circuits when power is restored to the device. Alternative solutions to avoid losing information in volatile digital circuits, such as battery backup, also add cost and complexity to digital designs. 
         [0021]    Important characteristics for logic circuits in an electronic device are low cost, high density, low power, and high speed. Resistance to radiation and the ability to function correctly at elevated temperatures also expand the applicability of digital logic. Conventional logic solutions are limited to silicon substrates, but logic circuits built on other substrates would allow logic devices to be integrated directly into many manufactured products in a single step, further reducing cost. 
         [0022]    Recently, devices have been proposed which use nanoscopic wires, such as single-walled carbon nanotubes, to form crossbar junctions to serve as memory cells. (See WO 01/03208, Nanoscopic Wire-Based Devices, Arrays, and Methods of Their Manufacture; and Thomas Rueckes et al., “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing,” Science, vol. 289, pp. 94-97, 7 July, 2000.) Hereinafter these devices are called nanotube wire crossbar memories (NTWCMs). Under these proposals, individual single-walled nanotube wires suspended over other wires define memory cells. Electrical signals are written to one or both wires to cause them to physically attract or repel relative to one another. Each physical state (i.e., attracted or repelled wires) corresponds to an electrical state. Repelled wires are an open circuit junction. Attracted wires are a closed state forming a rectified junction. When electrical power is removed from the junction, the wires retain their physical (and thus electrical) state thereby forming a non-volatile memory cell. 
         [0023]    U.S. Patent Publication No. 2003-0021966 discloses, among other things, electromechanical circuits, such as memory cells, in which circuits include a structure having electrically conductive traces and supports extending from a surface of a substrate. Nanotube ribbons that can electromechanically deform, or switch are suspended by the supports that cross the electrically conductive traces. Each ribbon comprises one or more nanotubes. The ribbons are typically formed from selectively removing material from a layer or matted fabric of nanotubes. 
         [0024]    For example, a nanofabric may be patterned into ribbons, and the ribbons can be used as a component to create non-volatile electromechanical memory cells. The ribbon is electromechanically-deflectable in response to electrical stimulus of control traces and/or the ribbon. The deflected, physical state of the ribbon may be made to represent a corresponding information state. The deflected, physical state has non-volatile properties, meaning the ribbon retains its physical (and therefore informational) state even if power to the memory cell is removed. Three-trace architectures may be used for electromechanical memory cells, in which two of the traces are electrodes to control the deflection of the ribbon. 
         [0025]    The use of an electromechanical bi-stable device for digital information storage has also been suggested. 
         [0026]    The creation and operation of bi-stable, nano-electro-mechanical switches based on carbon nanotubes (including mono-layers constructed thereof) and metal electrodes has been detailed in previous patent applications of Nantero, Inc. (U.S. Pat. Nos. 6,574,130, 6,643,165, 6,706,402; U.S. patent application Ser. Nos. 09/915,093, 10/033,323, 10/033,032, 10/128,117, 10/341,005, 10/341,055, 10/341,054, 10/341,130, 10/776,059, 10/776,572, 10/917,794, and 10/918,085 the contents of which are hereby incorporated by reference in their entireties). 
       SUMMARY 
       [0027]    The present invention provides logic circuits having a mode wherein the output can be set to a high-impedance condition. In various embodiments, the logic circuit and/or the tri-state feature may be implemented using nanotube switching elements and associated structures. In preferred embodiments, the logic circuits are implemented using complementary logic, particularly carbon nanotube-based complementary logic. In certain embodiments, the circuits thus combine the benefits of nanotube switching elements with the tri-state feature. 
         [0028]    In certain embodiments, a pull-up tri-state element and a pull-down tri-state element are provided. The pull-up tri-state element is disposed between the upper power supply voltage and the pull-up logic circuitry. The pull-down tri-state element is disposed between the lower power supply voltage and the pull-down logic circuitry. Each tri-state element has a control structure for receiving a signal (or signals) that controls the activation of the element. The tri-state element is implemented using at least one nanotube-switching element. 
         [0029]    In one aspect of the invention, a tri-state logic circuit includes an input terminal for receiving an input signal and an output terminal for providing an output signal. It also includes a pull-up network for connecting the output terminal to an upper power supply voltage, which is responsive to the input signal. The circuit further includes a first tri-stating nanotube switching element electrically connected in the signal path between the upper power supply voltage and the output terminal. The circuit also includes a pull-down network for connecting the output terminal to a lower power supply voltage, which is responsive to the input signal. A second tri-stating nanotube switching element is electrically connected in the signal path between the lower power supply voltage and the output terminal. The first and second nanotube switching elements are activated and deactivated to a tri-state control signal and the output terminal is tri-stated when the first and second nanotube switching elements are not activated. 
         [0030]    In one aspect of the invention, the circuit implements a Boolean function. 
         [0031]    In another aspect of the invention, the circuit implements an inverting function. 
         [0032]    In another aspect of the invention, the pull-up network and the pull-down network are constructed of nanotube switching elements. 
         [0033]    In another aspect of the invention, the first and second tri-stating nanotube switching elements are volatile. 
         [0034]    In another aspect of the invention, the first and second tri-stating nanotube switching elements are non-volatile. 
         [0035]    In another aspect of the invention, the first and second tri-stating nanotube switching elements are four-terminal devices. 
         [0036]    In another aspect of the invention, a tri-state logic circuit, includes an input terminal for receiving an input signal and an output terminal for providing an output signal. A network of nanotube switching elements is connected between the input terminal and the output terminal such that it implements a Boolean transformation of the input signal to generate the output signal. A tri-stating nanotube switching element is connected to the network, activated by a tri-state control signal, and arranged so that the output terminal is tri-stated when the tri-stating nanotube switching element is not activated. 
         [0037]    In one aspect of the invention, the nanotube switching elements in the network are volatile. 
         [0038]    In another aspect of the invention, the nanotube switching elements in the network are non-volatile. 
         [0039]    In another aspect of the invention, the tri-stating nanotube switching element is volatile. 
         [0040]    In another aspect of the invention, the tri-stating nanotube switching element is non-volatile. 
         [0041]    In another aspect of the invention, the tri-stating nanotube switching element is a four-terminal device. 
         [0042]    In another aspect of the invention, an inverter circuit includes a dual-rail differential input, for receiving a first input signal and a first complementary input signal, and a dual-rail differential output, for providing a first output signal and a first complementary output signal. The circuit further includes a first inverter for inverting the first input signal to generate said first output signal, and a first tri-stating nanotube switching element pair connected to said first inverter. The circuit further includes a second inverter for inverting said first complementary input signal to generate said first complementary output signal, and a second tri-stating nanotube switching element pair connected to said second inverter. The circuit further includes a dual-rail differential control input, for receiving a first control input signal and a first complementary control input signal provided to cooperatively activate and deactivate the first tri-stating element pair and the second tri-stating element pair. The dual-rail differential output is in a floating state when the first tri-stating element pair and the second tri-stating element pair are deactivated. 
         [0043]    In another aspect of the invention, the first inverter and the second inverter are constructed from nanotube switching elements. 
         [0044]    In another aspect of the invention, the nanotube switching elements of the first inverter and the second inverter are volatile. 
         [0045]    In another aspect of the invention, the nanotube switching elements of the first inverter and the second inverter are non-volatile. 
         [0046]    In another aspect of the invention, the nanotube switching elements of the first and second tri-stating element pairs are volatile. 
         [0047]    In another aspect of the invention, the nanotube switching elements of the first and second tri-stating element pairs are non-volatile. 
         [0048]    The provision of a high-impedance state for the output allows the outputs of multiple inverters to be connected together to form busses, logic decoders, or other circuits. Certain embodiments may offer certain advantages. For example, there is no significant leakage current between input and output terminals in the “OFF” state of a complementary nanotube-based device, and there is no junction leakage. The nanotube devices may operate in harsh environments such as elevated temperatures, e.g., 150 to 200 deg-C. or higher. The nanotube devices do not exhibit alpha particle sensitivity. 
         [0049]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]      FIGS. 1A-C  and  2 A-C illustrate differential logic inverters according to certain embodiments of the invention; 
           [0051]      FIGS. 3A-D  are illustrations of an exemplary nanotube switching element used in certain embodiments of the invention; and 
           [0052]      FIGS. 4A-C  are schematic representations of a nanotube switching element in various states of operation according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0053]    The present invention provide tri-state nanotube logic circuits constructed from one or more nanotube-switching elements. The use of some embodiments of such devices offers a solution to the CMOS problems of power dissipation and radiation sensitivity. In certain embodiments, the present invention uses electromechanical switches including carbon nanotube channel elements to build complementary nanotube logic. Nanotube-based pull-up and pull-down circuits can be connected to form complementary circuits that only dissipate power when switching. In embodiments of the invention, the circuits are tri-state circuits that have a mode wherein the output can be set to a high-impedance condition. In certain embodiments, the circuits thus combine the benefits of nanotube switching elements with the tri-state feature. For complementary logic circuits, a pull-up tri-state element and a pull-down tri-state element are provided. In certain embodiments, the pull-up tri-state element is disposed between the upper power supply voltage and the pull-up tri-state logic circuitry. The pull-down element is disposed between the lower power supply voltage and the pull-down logical circuitry. Each tri-state element has a control structure for receiving a signal (or signals) that turns the element ON and OFF. The tri-state element is implemented using at least one nanotube-switching element. The provision of a high-impedance state for the output allows the outputs of multiple inverters to be connected together to form logic busses, logic decoders, or other logic circuits. There is no significant leakage current between input and output terminals in the “OFF” state of a complementary nanotube-based device, and there is no junction leakage. The nanotube devices may operate in harsh environments such as elevated temperatures, e.g., 150 to 200 deg-C. or higher. There is no alpha particle sensitivity. The interconnect wiring used to interconnect the nanotube device terminals may be conventional wiring such as AlCu, W, or Cu wiring with appropriate insulating layers such as SiO 2 , polyimide, etc, or may be single or multi-wall nanotubes used for wiring. 
         [0054]    Preferred embodiments of the invention provide device control inputs to switch the complementary logic outputs of an inverter from a low impedance logic zero and logic one state to a high impedance floating state. Outputs from multiple logic devices providing tri-state logic may be connected together, as long as only a single device is enabled to the low impedance state to drive the common output. The use of logic circuits providing a third, high impedance state to create bus structures and logic decoders is well known. The technology can be used in all present memory devices such as DRAM, SRAM, Flash, EEPROM, PROM, etc. Also, logic functions such as microprocessor, adders, random logic, PLAs, FPGAs, etc. can be fabricated with this invention. 
         [0055]    Aspects of the invention are demonstrated herein by reference to a device that implements an inverting logic function.  FIG. 1A  illustrates an exemplary differential (dual-rail input and dual-rail output) logic inverter  10 , in which aspects of the invention may be used. The inverter  10  is constructed from four non-volatile nanotube switches  24 ,  26 ,  34 , and  36 , arranged in two inverters, a first inverter  20  and a second inverter  30 .  FIGS. 1B and 1C  illustrate operation of inverter  10 . (The schematic notation is discussed below with respect to  FIGS. 4A-C .)  FIG. 2A  illustrates an exemplary tri-statable differential (dual-rail input and dual-rail output) logic inverter  50 . Tri-statable differential logic inverter  50  also includes inverters  20  and  30 , and each inverter  20  and  30  is connected to a pull-up tri-stating element provided by a nanotube switch  52 ,  62  and a pull-down tri-stating element provided by a nanotube switch  56 ,  66 . 
         [0056]    The operation of inverters  10  and  50  is better understood by reference to the operation of exemplary nanotube switching elements.  FIGS. 3A-D  depict an exemplary nanotube switching element  300  in cross-section and layout views and in two informational states for use in certain embodiments of the invention. A more detailed description of these switches and other architectures for nanotube switching elements may be found in the related cases identified and incorporated above. Non-volatile four-terminal nanotube switching elements are described in U.S. patent application Ser. No. 10/918,085, which is incorporated by reference above. A summary description follows here for convenience. 
         [0057]      FIG. 3A  is a cross sectional view of a preferred nanotube switching element  100 . Nanotube switching element includes a lower portion having an insulating layer  117 , control electrode  111 , and output electrodes  113   c,d . Nanotube switching element further includes an upper portion having release electrode  112 , output electrodes  113   a,b , and signal electrodes  114   a,b . A nanotube channel element  115  is positioned between and held by the upper and lower portions. 
         [0058]    Release electrode  112  is made of conductive material and is separated from nanotube channel element  115  by an insulating material  119 . The channel element  115  is separated from the facing surface of insulator  119  by a gap height G 102 . 
         [0059]    Output electrodes  113   a,b  are made of conductive material and are separated from nanotube channel element  115  by insulating material  119 . 
         [0060]    Output electrodes  113   c,d  are likewise made of conductive material and are separated from nanotube channel element  115  by a gap height G 103 . Notice that the output electrodes  113   c,d  are not covered by insulator. 
         [0061]    Control electrode  111  is made of conductive material and is separated from nanotube channel element  115  by an insulating layer (or film)  118 . The channel element  115  is separated from the facing surface of insulator  118  by a gap height G 104 . 
         [0062]    Signal electrodes  114   a,b  each contact the nanotube channel element  115  and can therefore supply whatever signal is on the signal electrode to the channel element  115 . This signal may be a fixed reference signal (e.g., V DD  or Ground) or varying (e.g., a Boolean discrete value signal that can change). Only one of the electrodes  114   a,b  need be connected, but both may be used to reduce effective resistance. 
         [0063]    Nanotube channel element  115  is a lithographically-defined article made from a porous fabric of nanotubes (more below). It is electrically connected to signal electrodes  114   a,b . The electrodes  114   a,b  and support  116  pinch or hold the channel element  115  at either end, and it is suspended in the middle in spaced relation to the output electrodes  113   a - d  and the control electrode  111  and release electrode  112 . The spaced relationship is defined by the gap heights G 102 -G 104  identified above. For certain embodiments, the length of the suspended portion of channel element  115  is about 300 to 350 nm. 
         [0064]    Under certain embodiments the gaps G 103 , G 104 , G 102  are in the range of 5-30 nm. The dielectric on terminals  112 ,  111 , and  113   a  and  113   b  are in the range of 5-30 nm, for example. The carbon nanotube fabric density is approximately 10 nanotubes per 0.2×0.2 um area, for example. The suspended length of the nanotube channel element is in the range of 300 to 350 nm, for example. The suspended length to gap ratio is about 5 to 15 to 1 for non-volatile devices, and less than 5 for volatile operation, for example. 
         [0065]      FIG. 3B  is a plan view or layout of nanotube switching element  100 . As shown in this figure, electrodes  113   b,d  are electrically connected as depicted by the notation ‘X’ and item  102 . Likewise electrodes  113   a,c  are connected as depicted by the ‘X’. In preferred embodiments the electrodes are further connected by connection  120 . All of the output electrodes collectively form an output node  113  of the switching element  100 . 
         [0066]    Under preferred embodiments, the nanotube switching element  100  of  FIGS. 3A and 3B  operates as shown in  FIGS. 3C  and D. Specifically, nanotube switching element  100  is in an OPEN (OFF) state when nanotube channel element is in position  122  of  FIG. 3C . In such state, the channel element  115  is drawn into mechanical contact with dielectric layer  119  via electrostatic forces created by the potential difference between electrode  112  and channel element  115 . Output electrodes  113   a,b  are in mechanical contact (but not electrical contact) with channel element  115 . Nanotube switching element  100  is in a CLOSED (ON) state when channel element  115  is elongated to position  124  as illustrated in  FIG. 3D . In such state, the channel element  115  is drawn into mechanical contact with dielectric layer  118  via electrostatic forces created by the potential difference between electrode  111  and channel element  115 . Output electrodes  113   c,d  are in mechanical contact and electrical contact with channel element  115  at regions  126 . Consequently, when channel element  115  is in position  124 , signal electrodes  114   a  and  114   b  are electrically connected with output terminals  113   c,d  via channel element  115 , and the signal on electrodes  114   a,b  may be transferred via the channel (including channel element  115 ) to the output electrodes  113   c,d.    
         [0067]    By properly tailoring the geometry of nanotube switching element  100 , the nanotube switching element  100  may be made to behave as a non-volatile or a volatile switching element. By way of example, the device state of  FIG. 3D  may be made to be non-volatile by proper selection of the length of the channel element relative to the gap G 104 . (The length and gap are two parameters in the restoring force of the elongated, deflected channel element  115 .) Length to gap ratios of greater than 5 and less than 15 are preferred for non-volatile device; length to gap ratios of less than 5 are preferred for volatile devices. 
         [0068]    The nanotube switching element  100  operates in the following way. If signal electrode  114  and control electrode  111  (or  112 ) have a potential difference that is sufficiently large (via respective signals on the electrodes), the relationship of signals will create an electrostatic force that is sufficiently large to cause the suspended, nanotube channel element  115  to deflect into mechanical contact with electrode  111  (or  112 ). (This aspect of operation is described more fully in the incorporated patent references.) This deflection is depicted in  FIGS. 3D  (and  3 C). The attractive force stretches and deflects the nanotube fabric of channel element  115  until it contacts the insulated region  118  of the electrode  111 . The nanotube channel element is thereby strained, and there is a restoring tensile force, dependent on the geometrical relationship of the circuit, among other things. 
         [0069]    By using appropriate geometries of components, the switching element  100  then attains the closed, conductive state of  FIG. 3D  in which the nanotube channel  115  mechanically contacts the control electrode  111  and also output electrode  113   c,d . Since the control electrode  111  is covered with insulator  118  any signal on electrode  114  is transferred from the electrode  114  to the output electrode  113  via the nanotube channel element  115 . The signal on electrode  114  may be a varying signal, a fixed signal, a reference signal, a power supply line, or ground line. The channel formation is controlled via the signal applied to the electrode  111  (or  112 ). Specifically the signal applied to control electrode  111  needs to be sufficiently different in relation to the signal on electrode  114  to create the electrostatic force to deflect the nanotube channel element to cause the channel element  115  to deflect and to form the channel between electrode  114  and output electrode  113 , such that switching element  100  is in the CLOSED (ON) state. 
         [0070]    In contrast, if the relationship of signals on the electrode  114  and control electrode  111  is insufficiently different, then the nanotube channel element  115  is not deflected and no conductive channel is formed to the output electrode  113 . Instead, the channel element  115  is attracted to and physically contacts the insulation layer on release electrode  112 . This OPEN (OFF) state is shown in  FIG. 3C . The nanotube channel element  115  has the signal from electrode  114  but this signal is not transferred to the output node  113 . Instead, the state of the output node  113  depends on whatever circuitry it is connected to and the state of such circuitry. The state of output node  113  in this regard is independent of channel element voltage from signal electrode  114  and nanotube channel element  115  when the switching element  100  is in the OPEN (OFF) state. 
         [0071]    If the voltage difference between the control electrode  111  (or  112 ) and the channel element  115  is removed, the channel element  115  returns to the non-elongated state (see  FIG. 3A ) if the switching element  100  is designed to operate in the volatile mode, and the electrical connection or path between the electrode  115  to the output node  113  is opened. 
         [0072]    Preferably, if the switching element  100  is designed to operate in the non-volatile mode, the channel element is not operated in a manner to attain the state of  FIG. 3A . Instead, the electrodes  111  and  112  are expected to be operated so that the channel element  115  will either be in the state of  FIG. 3C  or  3 D. 
         [0073]    The output node  113  is constructed to include an isolation structure in which the operation of the channel element  115  and thereby the formation of the channel is invariant to the state of the output node  113 . Since in the preferred embodiment the channel element is electromechanically deflectable in response to electrostatically attractive forces, a floating output node  113  in principle could have any potential. Consequently, the potential on an output node may be sufficiently different in relation to the state of the channel element  115  that it would cause deflection of the channel element  115  and disturb the operation of the switching element  100  and its channel formation; that is, the channel formation would depend on the state of an unknown floating node. In the preferred embodiment this problem is addressed with an output node that includes an isolation structure to prevent such disturbances from being caused. 
         [0074]    Specifically, the nanotube channel element  115  is disposed between two oppositely disposed electrodes  113   b,d  (and also  113   a,c ) of equal potential. Consequently, there are opposing electrostatic forces that result from the voltage on the output node. Because of the opposing electrostatic forces, the state of output node  113  cannot cause the nanotube channel element  115  to deflect regardless of the voltages on output node  113  and nanotube channel element  115 . Thus, the operation and formation of the channel is made invariant to the state of the output node. 
         [0075]    Under certain embodiments of the invention, the nanotube switching element  100  of  FIG. 3A  may be used as a pull-up or pull-down device to form power-efficient circuits. Unlike MOS and other forms of circuits, in complementary circuits, the nanotube based pull-up and pull-down devices may be identical devices and need not have different sizes or materials. To facilitate the description of such circuits and to avoid the complexity of the layout and physical diagrams of  FIGS. 3A-D , a schematic representation shown in  FIGS. 4A-C  has been developed to depict the switching elements. The nodes identified by the same reference numerals in  FIGS. 4A-C  correspond to the structures shown in  FIGS. 3A-D . The thick black line  204  represents the nanotube channel element and more particularly its contact state. In  FIG. 4B , the nanotube channel element is insulated from the output terminal and the device is OFF. In  FIG. 4C , the nanotube channel element is in electrical contact with the output terminal and the device is ON. 
         [0076]    In summary, a four-terminal nanotube switching element includes a nanotube channel element that provides a controllably formable conductive channel from an input terminal to an output terminal. The input terminal is permanently in electrical contact with the channel element. The input terminal is connected to an input signal that is preferably fixed or has a known potential. A control input provided via a control terminal controls the formation of the conductive channel. A release input, which is complementary to the control input in preferred embodiments, provided via a release terminal resets the nanotube channel element from an ON state to an OFF state. 
         [0077]    Referring again to  FIGS. 1A and 2A , inverter  20  has a first logical input A applied to input terminal  22 , a second logical input A C  applied via terminal  32 , and a logical output Aout provided on output terminal  28 . The control electrodes of switching elements  34  and  36  are tied together to input terminal  22 . The release electrodes of switching elements  24  and  26  are tied together to input terminal  32 . In preferred embodiments, A C  is the logical complement of signal A. The control and release electrodes are thus operated in a complementary fashion, ensuring that each nanotube switching element is in a known state during operation of the device  10 . The signal electrode of nonvolatile device  24  is connected to voltage V DD  (the upper power supply voltage) and the signal electrode of nonvolatile device  26  is connected to ground (the lower power supply voltage). In operation, a nanotube switching element, having the architecture used in preferred embodiments of the invention for switching elements  24 ,  26 ,  34  and  36 , inherently implements an inverting function. The switching element is activated by a potential difference between the signal electrode and the control and/or release electrode. Switching elements  24  and  26  are arranged to invert signal A. Only one of switches  24  and  26  will be conducting for a given value of differential signal A/A C , and output  28  will be connected to either V DD  (when A is logically zero) or GND (when A is logically one). 
         [0078]    Inverter  30  has a first logical input A C  applied to input terminal  32 , a second logical input A applied via terminal  22 , and a logical output Aout C  provided on output terminal  38 . The control electrodes of switching elements  34  and  36  are tied together to input terminal  32 . The release electrodes of switching elements  34  and  36  are also tied together to input terminal  22 . The signal electrode of switching element  34  is connected to voltage V DD , and the signal electrode of switching element  36  is connected to ground. Together, switching elements  34  and  36  operate like switching elements  24  and  26  to invert the input, but switching elements  34  and  36  are connected to logical input A C . Switching elements  34  and  36  are arranged to invert signal A C . Combined first and second inverters  20  and  30 , and associated interconnections and supply voltages, comprise differential inverter  10 . 
         [0079]      FIG. 1B  illustrates inverter state of inverter  10  when input A is positive (V DD ) and input A C  is ground. In switching element  24 , the nanotube element is attracted to the insulated release electrode and does not conduct. In switching element  26 , the nanotube element is attracted by the control electrode to contact the output electrode, forming a conductive path from GND to output terminal  28 . In switching element  34 , the nanotube switching element is likewise attracted by the control electrode to contact the output electrode, forming a conductive path from V DD  to output terminal  38 . In switching element  36 , the nanotube element is attracted to the insulated release electrode and does not conduct. In summary, inverter output  28  is at 0 volts, connected to ground by switch  26 , and inverter output  38  is positive, connected to V DD  by switch  34 . 
         [0080]      FIG. 1C  illustrates inverter  10  in a second state when input A is zero and input A C  is positive (V DD ). In switching element  24 , the nanotube element is attracted by the control electrode to contact the output electrode, forming a conductive path from V DD  to output terminal  28 . In switching element  26 , the nanotube element is attracted to the insulated release electrode and does not conduct. In switching element  34 , the nanotube element is attracted to the insulated release electrode and does not conduct. In switching element  36 , the nanotube switching element is likewise attracted by the control electrode to contact the output electrode, forming a conductive path from GND to output terminal  38 . In summary, inverter output  28  is at 0 volts, connected to ground by switch  26 , and inverter output  38  is positive, connected to V DD  by switch  34 . In operation, output Aout of inverter  10  is the logical inversion of input A and output Aout C  of inverter  10  is the logical inversion of input A C  (or, in other words, is equivalent to signal A). 
         [0081]      FIG. 2A  illustrates a preferred embodiment of a differential logic tri-state inverter circuit  50  formed of inverters  20  and  30  illustrated in  FIG. 1A , and non-volatile nanotube switching elements  52 ,  56 ,  62  and  66  and associated interconnections. Switching elements  52 ,  56 ,  62  and  66  are controlled by a signal C 1  and its complement C 1   C . Switching elements  52 ,  56 ,  62  and  66  are activated only when C 1  is high. Otherwise, switching elements  52 ,  56 ,  62  and  66  are in the OFF state, and in this condition, inverter  50  is tri-stated and the output is effectively disconnected, regardless of the values of input signal A and A C . 
         [0082]    Switching element  52  is electrically disposed between the power supply voltage and the pull-up circuit of inverter  20 . Switching element  52  is controlled by a tri-state control input  53  and tri-state release input  54 . Tri-state control input  53  and tri-state release input  54  are connected to complementary tri-state control signals C 1   C  and C 1 , respectively. Because switching element  52  is connected to V DD , switching element  52  is activated when the signal on input  53  is low, and it must be controlled by C 1   C , a signal with inverse polarity to C 1 , to obtain the desired operation. Switching element  56  is electrically disposed between the GND connection and the pull-down circuit of inverter  20 . 
         [0083]      FIG. 2B  illustrates tri-state inverter  50  in a first physical and electrical state nanotube switching elements  52 ,  56 ,  62  and  66  are in the “OFF” state, with the nanotube elements in contact with insulated opposing output electrodes. The signal electrodes of nanotube switching elements  24  and  26  and  34  and  36  are not connected to power supply lines. Accordingly, V DD  and ground voltages are not applied to the inverter devices and no inverter operation takes place. Output voltages of signals Aout and Aout C  are not defined. 
         [0084]    The tri-stating function operates like an enable/disable feature. The tri-stated output allows the circuit  50  to share the same signal path as other circuits. Tri-state inverter  50  has a number of applications, such as sharing a common bus (not shown) with other circuits. When tri-state inverter  50  is tri-stated, or in the “OFF” state, bus operation is controlled by other circuits (not shown). 
         [0085]      FIG. 2C  illustrates tri-state inverter  50  in a second physical and electrical state nanotube switching elements  52 ,  56 ,  64 , and  66  are in the “ON” state, with the nanotube elements in contact with corresponding output electrodes. Power supply voltage V DD  is applied to nanotube switching elements  24  and  34 , and ground is applied to nanotube switching elements  26  and  36 . With tri-state inverter in the “ON” state, inverter operation as described with respect to  FIG. 1  resumes. In the “ON” state, tri-state inverter  100  controls (drives) a shared bus (not shown) or other circuits interconnected to the differential outputs  28  and  38 . 
         [0086]    The nanotube switching elements of preferred embodiments are non-volatile. Inverter  50  thus can retain both its logical state and its enable state if power to the circuit is removed or interrupted. The original state is present when power to the circuit is resumed. This property has a number of applications and advantages, including power failure protection and memory functions. 
         [0087]    The following are assigned to the assignee of this application, and are hereby incorporated by reference in their entirety:
       U.S. patent application Ser. No. 10/341,005, filed on Jan. 13, 2003, entitled Methods of Making Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements and Articles;   U.S. patent application Ser. No. 09/915,093, now U.S. Pat. No. 6,919,592, filed on Jul. 25, 2001, entitled Electromechanical Memory Array Using Nanotube Ribbons and Method for Making Same;   U.S. patent application Ser. No. 10/033,032, now U.S. Pat. No. 6,784,028, filed on Dec. 28, 2001, entitled Methods of Making Electromechanical Three-Trace Junction Devices;   U.S. patent application Ser. No. 10/033,323, now U.S. Pat. No. 6,911,682, filed on Dec. 28, 2001, entitled Electromechanical Three-Trace Junction Devices;   U.S. patent application Ser. No. 10/128,117, now U.S. Pat. No. 6,835,591, filed on Apr. 23, 2002, entitled Methods of NT Films and Articles;   U.S. patent application Ser. No. 10/341,055, filed Jan. 13, 2003, entitled Methods of Using Thin Metal Layers to Make Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements and Articles;   U.S. patent application Ser. No. 10/341,054, filed Jan. 13, 2003, entitled Methods of Using Pre-formed Nanotubes to Make Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements and Articles;   U.S. patent application Ser. No. 10/341,130, filed Jan. 13, 2003, entitled Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements and Articles;   U.S. patent application Ser. No. 10/776,059, now U.S. Pat. No. 7,259,410, filed Feb. 11, 2004, entitled Devices Having Horizontally-Disposed Nanofabric Articles and Methods of Making The Same;   U.S. patent application Ser. No. 10/776,572, now U.S. Pat. No. 6,924,538, filed Feb. 11, 2004, entitled Devices Having Vertically-Disposed Nanofabric Articles and Methods of Making the Same;       
 
         [0098]    Preferred embodiments of the invention are compatible with CMOS circuits and can be manufactured in an integrated way with CMOS circuits. It is contemplated that certain embodiments of the invention can be used interchangeably with existing field effect device implementations, e.g., CMOS implementations. CMOS designs can readily be converted to nanotube switch designs. Storage devices constructed according to the invention can be substituted for CMOS cells in larger CMOS circuits without impacting other portions of the original design. New designs combining nanotube switch technology with CMOS technology can readily be created by using embodiments of present invention with components selected from a CMOS device library. 
         [0099]    In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the present invention. Preferred embodiments use the nanotube-based switches of the incorporated related references. As described therein, many volatile and non-volatile configurations may be used. Combinations of different configurations may also be used. These switches may then be arranged and sized based on the requirements of a particular application, limitations of certain fabrication techniques, etc. While single walled carbon nanotube channel elements are preferred, multi-walled carbon nanotubes may also be used. Also, nanotubes may be used in conjunction with nanowires. Nanowires as referenced herein includes single nanowires, aggregates of non-woven nanowires, nanoclusters, nanowires entangled with nanotubes comprising a nanofabric, mattes of nanowires, etc. While carbon nanotube channel elements are preferred, it is contemplated that other nanotube channel elements may also be used in some embodiments. 
         [0100]    Other aspects, modifications, and embodiments are within the scope of the following claims. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.