Abstract:
CMOS semiconductor pass-transistor logic circuitry ( 200 ) is disclosed, comprising pass transistor circuitry ( 204, 212, 218 ), and tunneling structure circuitry ( 228 ) coupled to the pass transistor circuitry; where the tunneling structure circuitry is adapted to hold a node ( 222 ) voltage stable by compensating a leakage current ( 302 ) originating from said pass transistor circuitry.

Description:
This application claims priority under 35 USC §119 (e) (1) of Provisional Application No. 60/144,124, filed Jul. 16, 1999. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates, in general, to logic circuitry used in electronic devices, and in particular, to pass transistor logic circuitry designed for a Complementary Metal Oxide Semiconductor (CMOS) process including quantum mechanical tunneling structures. 
     BACKGROUND OF THE INVENTION 
     The continual demand for enhanced transistor and integrated circuit performance has resulted in improvements in existing devices, such as silicon, bipolar, and CMOS transistors and Galium Arsenide (GaAs) transistors, and also in the introduction of new device types and materials. In particular, scaling down device sizes to enhance high frequency performance leads to observable quantum mechanical effects, such as carrier tunneling through potential barriers. These effects led to development of alternative device structures which take advantage of such tunneling phenomenon; such as tunneling, and resonant tunneling, diodes and transistors. For ease of reference, all such structures are hereafter collectively referred to as tunneling diodes (TDs). 
     Tunneling diodes are generally two terminal devices with conduction carriers tunneling through potential barriers to yield current-voltage curves with portions exhibiting negative differential resistance (NDR). This negative differential resistance characteristic has been used as the basis for a wide range of high performance designs. 
     Conventionally, tunneling and resonant tunneling diodes have been limited in implementation to GaAs and other high performance processes. Conventional methods have focused on building TDs in GaAs for several reasons; mainly because the speed characteristics and small process features of GaAs processes were conducive to tunneling mechanics. However, performance considerations such as difficulty controlling peak current in TDs, limited their practical application and use. Additionally, since GaAs processes were not practical or cost efficient for high-volume, consumer-related production, TDs were generally limited in application to research and developmental applications. 
     Previously, the feature size of standard silicon processes, such as CMOS, was not conducive to producing such tunneling structures. Other conventional methods of utilizing tunneling structures in conjunction with standard silicon processes entailed fabrication of a TD structure in a non-silicon process, followed by transferring and bonding (or electrically coupling) the TD structure to a host silicon substrate. While certain performance issues may have thus been addressed, such a process required extra design time and processing steps. The additional design and fabrication costs associated with these approaches is therefore not commercially viable for large volume logic device production. 
     Thus, conventional implementations of tunneling structures have been used only in discrete form and niche applications, such as high speed pulse and edge generation; produced in costly, high-performance processes. Limitations to conventional tunneling structures include the difficulty in controlling peak current and the lack of an integrated circuit process capable of commercially producing tunneling structures in a commercially viable format. 
     In the absence of commercially viable TDs, conventional CMOS logic circuit designs have utilized functional components readily available in the CMOS process, such as inverters and logic and transmission gates. Conventional methods have focused on optimizing the design of these components individually, and improving their efficiency when utilized within larger circuits. Such conventional methods inevitably yield device inefficiency; due mainly to layout area, power consumption, and operational speed limits resulting from standard CMOS components. 
     As performance demands have increased and feature sizes for CMOS processes have decreased, fabrication of tunneling structures in a production CMOS process becomes feasible. Tunnel diode growth on silicon is relatively immature. Recently, CMOS compatible tunnel diodes have been demonstrated to show that a wide range of current densities can be obtained; addressing requirements for imbedded memory and signal processing applications. 
     Therefore, a system of logic circuitry designs incorporating tunneling structures for a CMOS process is now needed; providing enhanced design performance and efficiency while overcoming the aforementioned limitations of conventional methods. 
     SUMMARY OF THE INVENTION 
     Pass-transistor logic (PTL), being more compact than other CMOS logic implementations, has been widely used in applications that demand high-speed, high-density, and low-power. PTL is typically employed to provide connectivity between large functional blocks of circuitry, and is especially useful in fast multiplication applications. As such, PTL dominates the design of adders, multipliers, ALU&#39;s, signal processing units, and circuitry performing Boolean operations. It is widely used in address decoders and read circuits for memories; as well as control circuitry. 
     In the present invention, PTL circuitry is designed for a CMOS process including quantum mechanical tunneling structures; providing circuit layout area, power consumption, and operational speed advantages over conventional methods. NDR and current-voltage (I-V) characteristics of tunneling structures are exploited to provide high-performance, high functionality logic circuitry. Tunneling structures are utilized, replacing conventional CMOS components, to address MOS leakage and hold data state in a PTL circuit. 
     In one embodiment of the present invention, a pass-transistor network is designed, incorporating a tunneling diode, to implement a Boolean operation. The tunneling diode replaces a number of components used in conventional designs, providing high system performance with optimum design overhead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a schematic of a prior art CMOS pass-transistor network circuit; 
     FIG. 2 is a schematic of a CMOS pass-transistor network circuit incorporating a tunneling diode according to the present invention; 
     FIG. 3 is a graph illustrating characteristics of the circuit of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     The present invention defines logic circuitry employing tunneling structures in a homogenous silicon process; providing increased performance and design optimization. The present invention provides reduced circuit complexity for pass-transistor logic (PTL) circuits; decreasing the number of circuit components used, the number of interconnects, and the number of delay stages. The present invention thus realizes a significant reduction in layout area, operation delay, and power consumption over conventional methods. NDR and current-voltage (I-V) characteristics of tunneling structures are exploited to provide high-performance, high functionality logic circuitry. 
     For purposes of illustration, pass-transistor network circuitry utilizing resonant tunneling diodes (RTDs) is provided. However, the principles and applications of the present invention are applicable to resonant tunneling diodes as well as Esaki (p + n + ) diodes; hereafter collectively referred to as tunneling diodes (TDs). TDs are desirable for use in high speed logic circuit applications due to the fact that their switching speed is generally faster than standard MOS structures. TDs are well-known for their intrinsic bi-stability and high-speed switching capability due to their negative differential resistance (NDR) characteristic. High current density, low capacitance, and the NDR of TDs make them lo very fast non-linear circuit elements. These same device characteristics can be exploited in high-speed, low-power, digital logic circuits. Thus, in most general purpose applications, where a large fraction of the circuits may be idle at any given time, the present invention provides a significant advantage over conventional methods because CMOS structures have lower tunnel power dissipation due to very low static power consumption. 
     Referring now to FIG. 1, a schematic representative of a prior art pass-transistor logic circuit  100  is shown. Circuit  100  implements the Boolean condition  102  shown with the circuit. 
     Pass transistor  104  has base input  106 , representing “B”, and input  108 , representing “C”, at one end. Transistor  104  couples at its other end to node  110 . Similarly, pass transistor  112  has base input  114 , representing “D”, input  116 , representing “E”, at one end, and is coupled at its other end to node  110 . Pass transistor  118  has base input  120 , representing “A”, and couples at one end to node  110  and at its other end to node  122 . 
     Transistor  124  is coupled at one end to node  122 ; having its other end coupled to V DD  and its base  126  coupled to a pre-charge voltage. Similarly, transistor  128  is coupled at one end to node  122  and its other end to V DD ; having its base coupled to node  130 . Additionally, inverter  132  is coupled between nodes  122  and  130 . 
     In operation, node  130  is pre-charged, via transistor  124 , to a high voltage level “1”. If condition  102  is satisfied, node  130  will transition low to a “0” level. Due to MOS leakage current associated with transistors  104 ,  112 , and  118 , however, node  130  may transition erroneously. Transistor  128  and inverter  132  effectively form a loop used to address MOS leakage current effects on the node  130  voltage. 
     In contrast, FIG. 2 illustrates a pass-transistor logic circuit  200  according to the present invention. Circuit  200  implements Boolean condition  202 . Pass transistor  204  is an nMOS transistor having base input  206 , representing “B”, and input  208 , representing “C”, at one end. Transistor  204  couples at its other end to node  210 . Similarly, pass transistor  212  has base input  214 , representing “D”, input  216 , representing “E”, at one end, and couples at its other end to node  210 . Pass transistor  218  has base input  220 , representing “A”, and couples at one end to node  210  and at its other end to node  222 . Node  222  provides- the output for circuit  200 ; the evaluation of condition  202 . Pre-charge transistor  224  is a pMOS transistor, coupling one end to node  222 , coupling at its other end to V DD  and at its base  226  to a pre-charge voltage. Tunneling diode (TD)  228  couples at one end to node  222  and its other end to V DD . 
     In operation, node  222  is pre-charged, via transistor  224 , to a high voltage level “1”. If condition  202  is satisfied, node  222  will discharge to a low (“0”) level through transistors  204 ,  212 , and  218 . If condition  202  is not satisfied, node  222  should remain at a high level. Due to MOS leakage currents associated with transistors  204 ,  212 , and  218 , however, node  222  may transition erroneously. TD  228  provides a state hold functionality, however, compensating for these leakage currents. Thus, node  222  voltage level remains stable and TD  228  provides a high reliability system. 
     FIG. 3 illustrates one advantage of the present invention. A current-voltage plot  300  shows critical characteristics of circuit  200 . 
     Referring now jointly to FIGS. 2 and 3, curve  302  represents the collective leakage current effects of transistors  204 ,  212 , and  218 , while curve  304  represents the current-voltage characteristic of TD  228 . Equilibrium state  306  represents the point at which the TD current equals the leakage current. TD  228  thus compensates for the leakage current; the high peak current of TD  228  holding the voltage at node  222  at a high level. Errors resulting from leakage current are thus eliminated. The present invention thus provides high stability and overall design reliability. 
     With the present invention, a tunneling diode structure eliminates unnecessary inverter and transistor circuitry from a pass-transistor network circuit. This results in the CMOS/TD pass-transistor logic circuit of the present invention having substantially smaller area, substantially higher speed, and substantially lower power consumption in comparison with conventional CMOS counterparts. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.