Patent Publication Number: US-7720514-B2

Title: Receiver circuit using nanotube-based switches and logic

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 11/033,215, filed on Jan. 10, 2005, entitled Receiver Circuit Using Nanotube-Based Switches and Logic, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Pat. Apl. Ser. No. 60/581,075, filed on Jun. 18, 2004, entitled Non-Volatile Carbon Nanotube Logic (NLOGIC) Receiver Circuit, which is incorporated herein by reference in its entirety. 
     This application is related to the following references:
         U.S. Pat. No. 7,115,960, filed on Aug. 13, 2004, entitled Nanotube-Based Switching Elements;   U.S. Pat. No. 6,990,009, filed on Aug. 13, 2004, entitled Nanotube-Based Switching Elements With Multiple Controls;   U.S. Pat. No. 7,071,023, filed on Aug. 13, 2004, entitled Nanotube Device Structure And Methods Of Fabrication;   U.S. Pat. No. 7,138,832, filed on Aug. 13, 2004, entitled Nanotube-Based Switching Elements And Logic Circuits;   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. 11/033,087, filed on Jan. 10, 2005, entitled, Nanotube-Based Transfer Devices and Related Circuits;   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. Pat. No. 7,164,744, filed on Jan. 10, 2005, entitled, Nanotube-based Logic Driver Circuits;   U.S. Pat. No. 7,161,403, filed on Jan. 10, 2005, entitled, Storage Elements Using Nanotube Switching Elements;   U.S. Pat. No. 7,167,026, filed on Jan. 10, 2005, entitled, Tri-State Circuit Using Nanotube Switching Elements; and   U.S. patent application Ser. No. not yet assigned, filed on date even herewith, entitled Receiver Circuit Using Nanotube-Based Switches and Transistors.       

    
    
     BACKGROUND 
     1. Technical Field 
     The present application generally relates to nanotube switching circuits and in particular to nanotube switching circuits used in receiver circuits. 
     2. Discussion of Related Art 
     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. 
     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. 
     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. 
     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. 
     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 Jul. 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. 
     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. 
     For example, as disclosed in U.S. Patent Publication No. 2003-0021966, 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. As explained in U.S. Patent Publication No. 2003-0124325, three-trace architectures may be used for electromechanical memory cells, in which the two of the traces are electrodes to control the deflection of the ribbon. 
     The use of an electromechanical bi-stable device for digital information storage has also been suggested (c.f. U.S. Pat. No. 4,979,149: Non-volatile memory device including a micro-mechanical storage element). 
     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 a previous patent application 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, and 10/776,572, the contents of which are hereby incorporated by reference in their entireties). 
     SUMMARY 
     The invention provides receiver circuits using nanotube based switches and logic. 
     Under one aspect of the invention, a receiver circuit includes a differential input having a first and second input link, and a differential output having a first and second output link. First, second, third and fourth switching elements each have an input node, an output node, a nanotube channel element, and a control structure disposed in relation to the nanotube channel element to controllably form and unform an electrically conductive channel between said input node and said output node. The control structure of the first switching element is in electrical communication with the first input link, and the input node is in electrical communication with a low reference voltage. The output node is in electrical communication with the first output link. The control structure of the second switching element in electrical communication with the second input link, and the input node is in electrical communication with a low reference voltage, and the output node is in electrical communication with the second output link. The output node of the third switching element is in electrical communication with the first output link, and the control structure is in electrical communication with the second output link and the input node is in electrical communication a high reference voltage. The output node of the fourth switching element is in electrical communication with the second output link, and the control structure is in electrical communication with the first output link and the input node is in electrical communication a high reference voltage. 
     Under another aspect of the invention, the control structure of the first and second switching elements includes a control (set) electrode and a release electrode, and the first input link is coupled to the control (set) electrode of the first switching element and the release electrode of the second switching element. The second input link is coupled to the control (set) electrode of the second switching element and the release electrode of the first switching element. 
     Under another aspect of the invention, the control structure of the third and fourth switching elements includes a control (set) electrode and a release electrode, and the first output link is coupled to the control (set) electrode of the fourth switching element. The second output link is coupled to the control (set) electrode of the third switching element, and the release electrodes of the third and fourth switching elements are coupled to the high reference voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a receiver circuit according to certain embodiments of the invention; 
         FIGS. 2A-D  illustrate nanotube switches as used in certain embodiments of the invention; 
         FIGS. 3A-C  depict the notation used to describe the nanotube switch and its states; and 
         FIGS. 4A-B  depict the operation of the receiver circuit shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of the invention provide a receiver circuit that uses nanotube-based switches. Preferably, the circuits are dual-rail (differential). The receiver circuit can sense small voltage inputs and convert them to larger voltage swings. 
       FIG. 1  depicts a preferred receiver circuit  10 . As illustrated the receiver circuit  10  receives differential input signal A T  and A C  on links  25  and  25 ′ and provides a differential signal to other logic  45  via links  32  and  32 ′. 
     Receiver  10  includes non-volatile nanotube switches  15  and  20 , and non-volatile nanotube switch pull-up devices  35  and  40 . The outputs  30  and  30 ′ of nanotube switches  15  and  20  are connected to the outputs of pull-up switches  35  and  40 . A T  is coupled to the control electrode (more below) of nanotube switch  15  and A C  is coupled to the release electrode (more below). A C  is coupled to the control electrode of nanotube switch  20  and A T  is coupled to the release electrode. Each nanotube switch  15  and  20  has its signal electrode (more below) coupled to ground. The outputs  30  and  30 ′ are cross-coupled to the control electrodes of the pull-up switches  35  and  40  as depicted. The release electrodes of each pull-up switch are tied to the nanotube channel element and signal electrode of the switch, as depicted. The signal electrode is tied to Vdd in this embodiment. The pull-up switches  35  and  40  are sized to be volatile devices. 
       FIGS. 2A-D  depict a preferred nanotube switching element  100  in cross-section and layout views and in two informational states. These switches may be used for switches  15  and  20  of  FIG. 1 . A more detailed description of these switches may be found in the related cases identified and incorporated above. A brief description follows here for convenience. 
       FIG. 2A  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 , 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. 
     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 . 
     Output electrodes  113   a,b  are made of conductive material and are separated from nanotube channel element  115  by insulating material  119 . 
     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. 
     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 . 
     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., Vdd 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. 
     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. 
     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. 
       FIG. 2B  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 . 
     Under preferred embodiments, the nanotube switching element  100  of  FIGS. 2A and 2B  operates as shown in  FIGS. 2C  and D. Specifically, nanotube switching element  100  is in an OPEN (OFF) state when nanotube channel element is in position  122  of  FIG. 1C . 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. 1D . 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.    
     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. 2D  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. 
     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 in the incorporated patent references.) This deflection is depicted in  FIGS. 2D  (and  2 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 tensil force, dependent on the geometrical relationship of the circuit, among other things. 
     By using appropriate geometries of components, the switching element  100  then attains the closed, conductive state of  FIG. 1D  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. 
     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. 2C . 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. 
     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. 2A ) 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. 
     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. 1A . 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. 2C  or  2 D. 
     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, an 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 the output 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. 
     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 equal but opposing electrostatic forces that result from the voltage on the output node. Because of the equal and 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. 
     Under certain embodiments of the invention, the nanotube switching element  100  of  FIG. 2A  may be used as pull-up and pull-down devices to form power-efficient circuits. Unlike MOS and other forms of circuits, the 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. 1A-D , a schematic representation has been developed to depict the switching elements. 
       FIG. 3A  is a schematic representation of a nanotube switching element  100  of  FIG. 2A . The nodes use the same reference numerals. The nanotube switching element  100  may be designed to operate in the volatile or non-volatile switching mode. In this example, a non-volatile switching mode is used as illustrated by switches  15  and  20  in  FIG. 1 . 
       FIGS. 3B-C  depict a nanotube channel element  100  when its signal electrodes is tied to ground, and its states of operation. For example,  FIG. 3B  is a schematic representation of the nanotube switching element in the OPEN (OFF) state illustrated in  FIG. 2C , in which node  114  and the nanotube channel element  115  are at ground, the control electrode  111  is at ground, and the release electrode  112  is at Vdd. The nanotube channel element is not in electrical contact with output node  113 , but instead is depicted by the short black line  203  representing the nanotube element contacting insulator  119 .  FIG. 3C  is a schematic representation of the switching element in the CLOSED (ON) state illustrated in  FIG. 2D . In this case, signal node  114  and the nanotube channel element  115  are at ground, the control electrode  111  is at Vdd, and the release electrode  112  is at ground. The nanotube channel element is deflected into mechanical and electrical contact with the output node  113 . Moreover, if as described above, geometries are selected appropriately, the contact will be non-volatile as a result of the Van der Waals forces between the channel element and the uninsulated, output electrode.) The state of electrical contact is depicted by the short black line  204  representing the nanotube channel element contacting the output terminal  113 . This results in the output node  113  assuming the same signal (i.e., Vdd) as the nanotube channel element  115  and signal node  114 . The switches  15  and  20  operate analogously but opposite when the signal electrode is tied to Vdd. 
     FIG.  3 A′ is a schematic representation of a nanotube switching element  100  of  FIG. 2A  designed to be used in a volatile operating mode with release electrode connected to the nanotube switching element through the switching node contacting the nanotube element as illustrated by switches  35  and  40  in  FIG. 1 . The nodes use the same reference numerals plus a prime (′). Also, the release electrode is electrically connected to the nanotube contact such that there is no voltage difference between release electrode and the nanotube channel element. The arrow is used to show the mechanical force and direction on the nanotube channel element  115 . For example, as depicted, the channel element has a bias away from electrode  111 , i.e., if the channel element  115  were deflected into contact with electrode  111  a mechanical restoring force would be in the direction of the arrow. 
     FIGS.  3 B′-C′ depict a nanotube channel element  100  when its signal electrodes are tied to VDD, and its states of operation. For example, FIG.  3 B′ is a schematic representation of the nanotube switching element in the OPEN (OFF) state illustrated in  FIG. 2C , in which node  114 ′ and the nanotube channel element  115 ′ are at VDD, the release electrode  112 ′ is electrically connected to node  114 ′ and is therefore also at VDD, and the control electrode  111 ′ is also at VDD. The nanotube channel element is not in electrical contact with output node  113 , but instead is in a non-extended position, restored by the mechanical restoring force indicated by the arrow in FIG.  2 B′. FIG.  3 C′ is a schematic representation of the switching element in the CLOSED (ON) state illustrated in  FIG. 2D . In this case, signal node  114 ′ and the nanotube channel element  115 ′ are at VDD, the release electrode  112 ′ is electrically connected to signal node  114 ′ and is therefore also at VDD, and the control electrode  111 ′ is at ground. The nanotube channel element is deflected into mechanical and electrical contact with the output node  113 . Moreover, if as described above, geometries are selected appropriately, the contact will be volatile and the channel element will remain in contact with the uninsulated output electrode until the electrostatic force is removed, and then the mechanical restoring force in the direction of the arrow will overcome the van der Waals forces and release nanotube channel element from contact with the output electrode. The state of the volatile electrical contact is depicted by the short black line  204 ′ representing the nanotube channel element contacting the output terminal  113 ′. This results in the output node  113 ′ assuming the same signal (i.e., Vdd) as the nanotube channel element  115 ′ and signal node  114 ′. The switches  35  and  40  operate analogously but opposite when the signal electrode is tied to ground. 
     Receiver  10  is designed with non-volatile nanotube switches  15  and  20 , and volatile nanotube switches  35  and  40 . Non-volatile switches  15  and  20  are designed such that the mechanical restoring forces that result from the nanotube elongation after switching are weaker than the van der Waals restraining forces. An electrostatic voltage is used (required) to change the state of the nanotube from “ON” (CLOSED) to “OFF” (OPEN), and “OFF” to “ON.” Volatile switches  35  and  40  have the release plate electrically connected to the nanotube contact so that there is no electrostatic restoring force. Volatile devices  35  and  40  are designed such that the mechanical restoring forces that result from the nanotube elongation after switching are stronger than the van der Waals restraining forces, and the volatile nanotube will return to from the “ON” state to the “OFF” state once the electrostatic field is removed (the difference voltage between the input electrode and the nanotube fabric goes to zero). The direction of the mechanical restoring force is indicated by an arrow in the symbol for volatile nanotube switches  35  and  40 . The nanotube contact of each of the non-volatile switches  15  and  20  is connected to ground (reference voltage VREF=0). 
       FIG. 4A  illustrates the operation of receiver  10  shown in  FIG. 1  when input voltage VAt=VRED, a positive voltage, and complementary voltage VAc=0. VRED is not necessarily the same as VDD, and may be lower than VDD, for example. The nanotube threshold voltage of nonvolatile nanotube switches  15  and  20  is set to activate the switches to the “ON” or “OFF” state in response to voltage VRED. That is, voltage difference of VRED or higher across the control node and nanotube channel element is sufficient to make the switch contact the output node and form a channel between the signal node and the output node. For the applied conditions illustrated in  FIG. 4A , the voltage difference between the input gate and the nanotube channel element of nonvolatile nanotube switch  15  forces the nanotube channel element in contact with the output electrode and output  30  is thus connected to ground (i.e., the voltage on the signal electrode of switch  15 ). Also, the voltage difference between release gate and the nanotube channel element of nonvolatile nanotube switch  20  forces the nanotube channel element in contact with the dielectric layer on the opposing output electrode, and output  30 ′ is in an open state. If volatile nanotube switch  40  is in the “ON” state at the time, a current will flow briefly from power supply VDD to ground through switches  40  and  15 . The resistance RNT of the nanotube channel element is chosen such that the RNT of switch  15  is substantially lower than RNT of switch  40  so that output  30  is held near ground voltage. RNT for switch  15  is chosen to be 3 to 5 time smaller than RNT for switch  40 . If switch  40  has a width of 10 parallel carbon nanotubes (NT fibers), then switch  15  is chosen to have a width of 30 to 50 parallel NT fibers, for example. When output  30  is forced to near zero volts, the input of switch  35  is forced to near zero volts and switch  35  turns “ON.” The input voltage of switch  40  transitions from zero to VDD, reducing the voltage difference between switch  40  input electrode and nanotube element to zero. As the electrostatic force between input electrode and nanotube goes to zero, the mechanical restoring force turns switch  40  “OFF” and current stops through switches  40  and  15 . Receiver  10  is in a state  10 ′ illustrated in  FIG. 4B . Logic gates  45  input  32  is at zero volts, and input  32 ′ is at VDD. Output  30 ′ is at VDD, but no current flows because switch  20  is in the “OFF” (OPEN) position (state). 
       FIG. 4B  illustrates the operation of receiver  10  shown in  FIG. 1  when input voltage VAt equals zero, and complementary voltage VAc=VRED, a positive voltage. VRED is not necessarily the same as VDD, and may be lower than VDD, for example. The nanotube threshold voltage of nonvolatile nanotube switches  15  and  20  is set to activate the switches to the “ON” or “OFF” state in response to voltage VRED. For the applied conditions illustrated in  FIG. 4B , the voltage difference between the input gate and the nanotube fabric of nonvolatile nanotube switch  20  forces the nanotube channel element in contact with the output electrode, and output  30 ′ is connected to ground. Also, the voltage difference between release gate and the nanotube channel element of nonvolatile nanotube switch  15  forces the nanotube channel element in contact with the dielectric layer on the opposing electrode, and output  30  is in an open state. If volatile nanotube switch  35  is in the “ON” state at the time, a current will flow briefly from power supply VDD to ground through switches  35  and  20 . Nanotube resistance RNT is chosen such that the RNT of switch  20  is substantially lower than RNT of switch  35  so that output  30 ′ is held near ground voltage. RNT for switch  20  is chosen to be 3 to 5 time smaller than RNT for switch  35 . If switch  35  has a width of 10 parallel NT fibers, then switch  20  is chosen to have a width of 30 to 50 parallel NT fibers, for example. When output  30 ′ is forced to near zero volts, the input of switch  40  is forced to near zero volts and switch  40  turns “ON.” The input voltage of switch  35  transitions from zero to VDD, reducing the voltage difference between switch  35  input electrode and nanotube element to zero. As the electrostatic force between input electrode and nanotube goes to zero, the mechanical restoring force turns switch  35  “OFF” and current stops through switches  40  and  15 . Receiver  10  is in a state  10 ″ illustrated in  FIG. 4B . Logic gates  45  input  32  is at VDD volts, and input  32 ′ is at zero. Output  30  is at VDD, but no current flows because switch  15  is in the “OFF” (OPEN) position (state). 
     Several of the incorporated, related patent references describe alternative variations of nanotube-based switches. Many of these may be incorporated into the embodiments described above, providing volatile or non-volatile behavior, among other things. Likewise the fabrication techniques taught in such cases may be utilized here as well. 
     Nanotube-based logic may be used in conjunction with and in the absence of diodes, resistors and transistors or as part of or a replacement to CMOS, biCMOS, bipolar and other transistor level technologies. Also, the non-volatile flip flop may be substituted for an SRAM flip flop to create a NRAM cell. 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 SiO2, polyimide, etc, or may be single or multi-wall nanotubes used for wiring. 
     There is no significant leakage current between input and output terminals in the “OFF” state of the nanotube-based switch, and there is no junction leakage. Therefore the nanotube-based switch may operate in harsh environments such as elevated temperatures, e.g., 150 to 200 deg-C. or higher. There is no alpha particle sensitivity. 
     While single walled carbon nanotubes are preferred, multi-walled carbon nanotubes may be used. Also nanotubes may be used in conjunction with nanowires. Nanowires as mentioned herein is meant to mean single nanowires, aggregates of non-woven nanowires, nanoclusters, nanowires entangled with nanotubes comprising a nanofabric, mattes of nanowires, etc. The invention relates to the generation of nanoscopic conductive elements used for any electronic application. 
     The following patent references refer to various techniques for creating nanotube fabric articles and switches and are assigned to the assignee of this application. Each is 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, 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, filed on Dec. 28, 2001, entitled Methods of Making Electromechanical Three-Trace Junction Devices;   U.S. patent application Ser. No. 10/033,323, filed on Dec. 28, 2001, entitled Electromechanical Three-Trace Junction Devices;   U.S. patent application Ser. No. 10/128,117, filed on Apr. 23, 2002, entitled Methods of NTFilms 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, filed Feb. 11, 2004, entitled Devices Having Horizontally-Disposed Nanofabric Articles and Methods of Making The Same; and   U.S. patent application Ser. No. 10/776,572, filed Feb. 11, 2004, entitled Devices Having Vertically-Disposed Nanofabric Articles and Methods of Making the Same.       

     Volatile and non-volatile switches, and switching elements of numerous types of devices, can be thus created. In certain preferred embodiments, the articles include substantially a monolayer of carbon nanotubes. In certain embodiments the nanotubes are preferred to be single-walled carbon nanotubes. Such nanotubes can be tuned to have a resistance between 0.2-100 kOhm/□ or in some cases from 100 kOhm/□ to 1 GOhm/□. 
     The receiver circuit facilitates compatibility between carbon nanotube logic circuits and CMOS logic. For example, the output of conventional CMOS circuits may drive nanotube-based switches. Dual-rail (differential) logic inputs are used and the receiver circuit may operate in a differential sensing mode, at smaller voltage swings for high speed and lower power dissipation, with no internal logic reference level needed at the receiving end. The output of the receiver circuit is a voltage selected for desired (e.g., optimum) on chip circuit operation. Consequently, the receiver circuit may operate at a different voltage than CMOS logic circuits. Preferred receiver circuits enable a nanotube logic chip or an embedded nanotube logic function using only nanotube logic to interface directly with CMOS circuits driving the receiver inputs, with input voltage signals that may be different from on chip voltage signals. Also, preferred receiver circuits enables integrated logic blocks using CMOS and combined nanotube-based logic and CMOS technologies to operate at different power supply voltages in the same system on separate chips, or integrated on the same chip. Such a receiver, and other combined circuits, may be used to facilitate the introduction of nanotube-based logic in a CMOS environment. 
     The nanotube switching element of preferred embodiments utilizes multiple controls for the formation and information of the channel. In some embodiments, the device is sized to create a non-volatile device and one of the electrodes may be used to form a channel and the other may be used to unform a channel. The electrodes may be used as differential dual-rail inputs. Alternatively they may be set and used at different times. For example, the control electrode may be used in the form of a clock signal, or the release electrode may be used as a form of clocking signal. Also, the control electrode and release electrode may be placed at the same voltage, for example, such that the state of the nanotube cannot be disturbed by noise sources such as voltage spikes on adjacent wiring nodes. 
     A  FIG. 2  device may be designed to operate as a volatile or non-volatile device. In the case of a volatile device, the mechanical restoring force due to nanotube elongation is stronger than the van der Waals retaining force, and the nanotube mechanical contact with a control or release electrode insulator is broken when the electrical field is removed. Typically, nanotube geometrical factors such as suspended length to gap ratios of less than 5 to 1 are used for volatile devices. In the case of a non-volatile device, the mechanical restoring force due to nanotube elongation is weaker than the van der Waals retaining force, and the nanotube mechanical contact with a control or release electrode insulator remains un-broken when the electric field is removed. Typically, nanotube geometrical factors such as suspended length to gap ratios of greater than 5 to 1 and less than 15 to 1 are used for non-volatile devices. An applied electrical field generating an electromechanical force is required to change the state of the nanotube device. Van der Waals forces between nanotubes and metals and insulators are a function of the material used in the fabrication nanotube switches. By way of example, these include insulators such as silicon dioxide and silicon nitride, metals such as tungsten, aluminum, copper, nickel, palladium, and semiconductors such as silicon. For the same surface area, forces will vary by less than 5% for some combinations of materials, or may exceed 2× for other combinations of materials, so that the volatile and non-volatile operation is determined by geometrical factors such as suspended length and gap dimensions and materials selected. It is, however, possible to design devices by choosing both geometrical size and materials that exhibit stronger or weaker van der Waals forces. By way of example, nanotube suspended length and gap height and fabric layer density, control electrode length, width, and dielectric layer thickness may be varied. Output electrode size and spacing to nanotube may be varied as well. Also, a layer specifically designed to increase van der Waals forces (not shown) may be added during the fabrication nanotube switching element  100  illustrated in  FIG. 1 . For example, a thin (5 to 10 nm, for example) layer of metal (not electrically connected), semiconductor (not electrically connected), or insulating material may be added (not shown) on the insulator layer associated with control electrode  111  or release electrode  112  that increases the van der Waals retaining force without substantial changes to device structure for better non-volatile operation. In this way, both geometrical sizing and material selection are used to optimize device operation, in this example to optimize non-volatile operation. 
     In a complementary circuit such as an inverter using two nanotube switching elements  100  with connected output terminals, there can be momentary current flow between power supply and ground in the inverter circuit as the inverter changes from one logic state to another logic state. In CMOS, this occurs when both PFET and NFET are momentarily ON, both conducting during logic state transition and is sometimes referred to as “shoot-through” current. In the case of electromechanical inverters, a momentary current may occur during change of logic state if the nanotube fabric of a first nanotube switch makes conductive contact with the first output structure before the nanotube fabric of a second nanotube switch releases conductive contact with the second output structure. If, however, the first nanotube switch breaks contact between the first nanotube fabric and the first output electrode before the second nanotube switch makes contact between the second nanotube fabric and the second output electrode, then a break-before-make inverter operation occurs and “shoot-through” current is minimized or eliminated. Electromechanical devices that favor break-before-make operation may be designed with different gap heights above and below the nanotube switching element, for example, such that forces exerted on the nanotube switching element by control and release electrodes are different; and/or travel distance for the nanotube switching element are different in one direction than another; and/or materials are selected (and/or added) to increase the van der Waals forces in one switching direction and weakening van der Waals forces in the opposite direction. 
     By way of example, nanotube switching element  100  illustrated in  FIG. 1  may be designed such that gap G 102  is substantially smaller (50% smaller, for example) than gap G 104 . Also, gap G 103  is made bigger such that nanotube element  115  contact is delayed when switching. Also, dielectric thicknesses and dielectric constants may be different such that for the same applied voltage differences, the electric field between release electrode  112  and nanotube element  115  is stronger than the electric field between control electrode  111  and nanotube element  115 , for example, to more quickly disconnect nanotube element  115  from output terminals  113   c  and  113   d . Output electrodes  113   c  and  113   d  may be designed to have a small radius and therefore a smaller contact area in a region of contact with nanotube element  115  compared with the size (area) of contact between nanotube element  115  and the insulator on control terminal  111  to facilitate release of contact between nanotube element  115  and output electrodes  113   c  and  113   d . The material used for electrodes  113   c  and  113   d  may be selected to have weaker van der Waals forces respect to nanotube element  115  than the van der Waals forces between nanotube element  115  and the insulator on release electrode  112 , for example. These, and other approaches, may be used to design a nanotube switching element that favors make-before-break operation thus minimizing or eliminating “shoot-through” current as circuits such as inverters switch from one logic state to another. 
     The material used in the fabrication of the electrodes and contacts used in the nanotube switches is dependent upon the specific application, i.e. there is no specific metal necessary for the operation of the present invention. 
     Nanotubes can be functionalized with planar conjugated hydrocarbons such as pyrenes which may then aid in enhancing the internal adhesion between nanotubes within the ribbons. The surface of the nanotubes can be derivatized to create a more hydrophobic or hydrophilic environment to promote better adhesion of the nanotube fabric to the underlying electrode surface. Specifically, functionalization of a wafer/substrate surface involves “derivitizing” the surface of the substrate. For example, one could chemically convert a hydrophilic to hydrophobic state or provide functional groups such as amines, carboxylic acids, thiols or sulphonates to alter the surface characteristics of the substrate. Functionalization may include the optional primary step of oxidizing or ashing the substrate in oxygen plasma to remove carbon and other impurities from the substrate surface and to provide a uniformly reactive, oxidized surface which is then reacted with a silane. One such polymer that may be used is 3-aminopropyltriethoxysilane (APTS). The substrate surface may be derivatized prior to application of a nanotube fabric. 
     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.