Patent Abstract:
A Boolean gate includes at least one symmetric tunneling field-effect transistor (SymFET) for low-power information processing. SymFETs are ideal for applications that demand low power and have moderate speed requirements, and demonstrate better dynamic energy efficiency than CMOS circuits. Negative differential resistance (NDR) behavior of SymFETs leads to hysteresis in inverters and buffers, and can be used to build simple Schmitt-triggers. Further, pseudo-SymFET loads may be utilized in circuits similar to all-n-type or dynamic logic. For example, latches and flip-flops as well as NAND, NOR, IMPLY, and MAJORITY gates may employ SymFETs. Such SymFET-based devices require fewer transistors than static CMOS-based designs.

Full Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to the field of low-power information processing hardware and, more specifically, to devices for utilizing symmetric tunneling field-effect transistors (SymFETs) for low-power information processing. 
     BACKGROUND OF RELATED ART 
     In the pursuit of low-power information processing hardware, many new post-complementary metal-oxide-semiconductor (CMOS) transistors have been created in the past few decades. One desirable property for these new devices is a large on-current to off-current ratio, which enables low-power, low-voltage operation. Initial studies and experiments on a class of emerging field effect transistor (FET) technologies that operate upon the principle of tunneling between 2-D materials have indicated their potential for low-power operation. However, current-voltage characteristics (or “I-V curves”) of these devices appear to be dissimilar to those of today&#39;s metal-oxide-semiconductor field-effect transistors (MOSFETs), and instead exhibit bell-curve characteristics. Further, double-layer graphene transistors, such as SymFETs, and bilayer pseudo-spin FETs (BiSFETs) may all possess at least some of the aforementioned characteristics. Such behavior is also observed in some molecular transistors and single-electron transistors. To date, though, little if any work has considered how such devices might be employed for the purposes of digital logic, which will ultimately help to determine the “fate” of these new device technologies. For example, scientists have not yet described how to leverage the use of SymFETs in sequential circuits. Therefore, it is both important and desirable to utilize new SymFET-based devices that are superior to CMOS designs with regards to compactness, speed, and/or energy efficiency. 
     Until now, little has been done in the way of designing circuits comprised of SymFETs and other devices with similar graphene-insulator-graphene structures. New topologies involving such devices can benefit from unique I-V curves. Known attempts to incorporate graphene-insulator-graphene structures to date have not proven helpful. For example, one recent gate design uses BiSFETs at a supply voltage of 25 millivolts (mV), but a 25 mV supply is likely to prove challenging when considering supply, substrate, and device noises, where, in practice, the ripple on supply rails often amounts to a few tens of mV. On the other hand, some researchers have proposed a static random-access memory (SRAM) cell without access transistors. Due to the negative differential resistance (NDR) of that device, the circuit is bistable while having only two transistors as opposed to four in a similar CMOS SRAM cell. 
     Therefore, a need exists for several new Boolean gates that are practical and take advantage of unique I-V curves. Even simple gates, such as an inverter, for example, exhibit new properties (i.e., hysteresis) when realized by way of SymFETs. The present disclosure concerns several new circuits with a topology different from those of popular CMOS designs. To quantify the performance of these new SymFET-based gates and determine their potential benefits, the present disclosure benchmarks these gates against existing CMOS-functional equivalents. The present disclosure also examines the relationship between SymFET and resonant tunneling diodes (RTDs) to verify the feasibility of adopting RTD-based circuits. In turn, the present disclosure will aid in developing new circuits with other SymFET-like transistors and also assist in improving future transistors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an exploded graphic perspective view of an example SymFET. 
         FIG. 1B  is a circuit diagram of the example SymFET of  FIG. 1A . 
         FIG. 2  is a chart plotting current against voltage for various voltages applied to top and back gates of the example SymFET of  FIGS. 1A-1B . 
         FIG. 3A  is a circuit diagram of an example SymFET-based inverter. 
         FIG. 3B  is chart showing voltage in/out signals of the example SymFET-based inverter of  FIG. 3A  when loaded by a fanout-of-4 (FO4) inverter at V DD =0.3 V. 
         FIGS. 4A-4C  together are a series of charts showing the variation in currents at first and second transistors T 1 , T 2  of the example SymFET-based inverter of  FIG. 3A  where V DD =0.3 V, with quiescent points shown by a star, along with a chart in  FIG. 4D  plotting the transfer characteristics of the SymFET-based inverter of  FIG. 3A . 
         FIGS. 5A-5C  together are a series of charts showing the variation in currents at first and second transistors T 1 , T 2  of the example SymFET-based inverter of  FIG. 3A  where V DD =0.5 V, with quiescent points shown by a star, along with a chart in  FIG. 5D  plotting the transfer characteristics of the SymFET-based inverter of  FIG. 3A . 
         FIG. 6A  shows a circuit diagram for an example Schmitt-trigger inverter. 
         FIG. 6B  shows a chart plotting the drain current of a first transistor T 1  of the example Schmitt-trigger inverter against the same current based on the example SymFET-based inverter of  FIG. 3A . 
         FIG. 6C  shows a chart indicating the transfer characteristics of the example Schmitt-trigger inverter for V DD =0.3 V. 
         FIG. 7A  is a circuit diagram of an example SymFET-based buffer. 
         FIG. 7B  is a chart plotting voltage out against voltage in for the example SymFET-based buffer of  FIG. 7A . 
         FIG. 8A  is a circuit diagram of another example SymFET-based buffer designed to eliminate or at least minimize hysteresis. 
         FIG. 8B  is a chart plotting voltage out against voltage in for the example SymFET-based buffer of  FIG. 8A . 
         FIG. 9A  is a circuit diagram of an example SymFET-based NAND gate. 
         FIG. 9B  is a circuit diagram of a pseudo-SymFET logic style gate. 
         FIG. 9C  is a chart plotting the I-V curve of the pseudo-SymFET logic style gate of  FIG. 9B , where a nonlinear load resistance was implemented by SymFET. 
         FIG. 9D  is a circuit diagram of an example pseudo-SymFET OR gate. 
         FIGS. 9E-9F  together show input and output charts, where the output of a pseudo-SymFET NOR gate is compared with the output of a gate with linear resistive load (i.e., V DD =0.4 V; size of a third transistor T 3  is twice that of first and second transistors T 1 , T 2 ; load is a FO4 inverter). 
         FIG. 10A  is a circuit diagram of a dynamic pseudo-SymFET NOR gate. 
         FIGS. 10B-10D  together show the simulated input/output of a NOR gate, where the size of T 3  is three times that of first and second transistors T 1 , T 2  and the load is a FO4 inverter. 
         FIG. 11A  is a circuit diagram for a SymFET-based IMPLY gate. 
         FIG. 11B  is a circuit diagram for a CMOS-based IMPLY gate. 
         FIG. 11C  shows a corresponding truth table applicable to both the SymFET- and CMOS-based IMPLY gates of  FIGS. 11A-11B . 
         FIGS. 11D-11F  together show several charts plotting input and output for the respective SymFET- and CMOS-based IMPLY gates of  FIGS. 11A-11B . 
         FIG. 12A  is a circuit diagram of an example NAND-based XOR gate. 
         FIG. 12B  is a circuit diagram of an example IMPLY-based XOR gate. 
         FIG. 13A  is a circuit diagram of an example resistive network, which serves as the basis for a MAJORITY gate. 
         FIG. 13B  is a circuit diagram of an example SymFET-based MAJORITY gate. 
         FIG. 13C  is a corresponding truth table for the example SymFET-based MAJORITY gate of  FIG. 13B . 
         FIGS. 13D-13E  together show an input voltage chart and an output voltage chart based on the example SymFET-based MAJORITY gate of  FIG. 13B . 
         FIG. 13F  is a chart plotting the absolute values of current versus voltage for a nonlinear SymFET-based resistor. 
         FIG. 13G  is a chart plotting total current versus voltage for the example SymFET-based MAJORITY gate of  FIG. 13B . 
         FIG. 14  is an exploded graphic perspective view of an example diode-connected SymFET. 
         FIG. 15  is a circuit diagram of an example full adder cell. 
         FIG. 16A  is a circuit diagram of an example SymFET-based reset/set (RS) latch. 
         FIG. 16B  is a model showing how two transistors of the example SymFET-based RS latch of  FIG. 16A  are being used. 
         FIG. 16C  is a chart plotting the current through a first and second transistor T 1 , T 2  of the example SymFET-based RS latch of  FIG. 16A  against output voltage when A=0 and B=V DD . 
         FIG. 16D  is a truth table of the example SymFET-based RS latch of  FIG. 16A . 
         FIG. 17A  is a circuit diagram of an RS latch with an inverter. 
         FIG. 17B  is a truth table for the RS latch of  FIG. 17A . 
         FIG. 17C  is a circuit diagram for a conventional RS latch having eight transistors. 
         FIGS. 18A-18B  show a simulated transient response of the example SymFET-based RS latch of  FIG. 16A . 
         FIG. 19A  is a circuit diagram of another example SymFET-based latch. 
         FIG. 19B  is a truth table for the example SymFET-based latch of  FIG. 19A . 
         FIGS. 19C-19E  together show several charts plotting current versus voltage at different input values for a latch without speedup transistors. 
         FIGS. 20A-20C  together show input and output voltages for latches with and without speedup. 
         FIGS. 21A-21C  together show the I-V curves of transistors from the example SymFET-based latch of  FIG. 19A  where I D13 =I DS1 +I DS3  and I S24 =I SD2 +I SD4 . 
         FIG. 22A  is a circuit diagram of an RTD-like device based on SymFETs. 
         FIG. 22B  is a chart plotting the sum of the currents in two SymFETs T 1 , T 2  of the device of  FIG. 22A  against input voltage. 
         FIG. 23A  is a circuit diagram of an RTD-based MOBILE inverter/DFF. 
         FIGS. 23B-23D  show several charts plotting current versus output voltage for a low A input as a clocked input gradually increases. 
         FIG. 24A  is a circuit diagram of an example SymFET-based MOBILE device. 
         FIG. 24B  is a chart plotting input through the SymFETs against output voltage of the SymFET-based MOBILE device of  FIG. 24A  for a high clock input. 
         FIG. 24C  is a chart similar to  FIG. 24B , but shows a combined current of I DS1 +I DS2 . 
         FIGS. 24D-24F  together show input/output waveforms of the example MOBILE circuit of  FIG. 24A  showing that the output F is high only if input A is low at the rising edge of the clocked input. 
         FIG. 25A  is a chart showing power dissipation of a SymFET-based inverter at 100 MHz with an FO4 inverter as load. 
         FIG. 25B  is a chart showing the propagation delay of a SymFET-based inverter measured from 50% input to 50% output. 
         FIG. 25C  is a chart illustrating the delay-energy relationship at different supply voltages. 
         FIG. 25D  is a chart showing energy dissipation of SymFET-based inverters (squares) in comparison with that of CMOS-based inverters at 100 MHz. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of example devices is not intended to limit the scope of the description to the precise form or forms detailed herein. Instead the following description is intended to be illustrative so that others may follow its teachings. 
     Device Description and Model 
       FIG. 1A  shows a physical structure of an example SymFET  100  along with an electrical diagram  102  in  FIG. 1B  representing the example SymFET  100 . In some examples, operation of the SymFET  100  shown in  FIGS. 1A-1B  is summarized as follows. Tunneling occurs between two example graphene layers  104 ,  106  separated by an example thin insulator  108 . The voltages associated with an example top-gate (TG)  110 , which is disposed above an example first oxide layer  112 , and an example back-gate (BG)  114 , which is disposed below an example second oxide layer  116 , change the carrier type/density of the graphene layers  104 ,  106  by electrostatic field. In the example, shown in  FIGS. 1A-1B , the graphene layer  104  is associated with a source  118 , while the graphene layer  106  is associated with a drain  120 . Because tunneling probability depends on occupied/empty states on both sides of the tunneling barrier, voltages of the top and back gates  110 ,  114  can modulate the drain-source current (I DS ). In addition,  FIG. 2  illustrates I-V characteristics  200  of the example SymFET  100  shown in  FIGS. 1A-1B  under various load conditions. Peak currents for each respective I DS —drain-source voltage (V DS ) curve depend on the voltages at the top and back gates  110 ,  114 . 
     To build a tabular representation of I DS  in terms of the terminal voltages for a device constructed with undoped graphene sheets, the present disclosure employs the previously-published analytical models described in P. Zhao et al., “SymFET: A Proposed Symmetric Graphene Tunneling Field-Effect Transistor,”  IEEE Trans. Electron Devices , vol. 60, no. 3, pp. 951-957 (March 2013), R. M. Feenstra et al., “Single-particle tunneling in doped graphene-insulator-graphene junctions,”  J. Appl. Phys ., vol. 111, no. 4, p. 043711 (April 2012), and S. C. de la Barrera et al., “Theory of graphene-insulator-graphene tunnel junctions,”  J. Vac. Sci. Technol. B , vol. 32, 04E101, (July/August 2014), all of which are hereby incorporated by reference in their entireties. The results are incorporated in a Verilog-A model such that the 4-terminal device can be used in simulation program with integrated circuit emphasis (SPICE) simulations. 
     In some examples, the device capacitances are constant and are based on planar capacitance between the TG  110  and the source  118 , between the BG  114  and the drain  120 , and between the drain  120  and the source  118 . Moreover, in the examples disclosed below, both the TG  110  and the BG  114  have an effective oxide thickness (EOT) of 1.2 nanometers (nm). In some examples, the thin insulator  108  acting as a tunneling barrier is comprised of 1.34 nm-thick Boron-Nitride (i.e., 4 layers of h-BN). Still further, coherence length determines what percentage of the graphene layers  104 ,  106  are structurally perfect. And in these examples, the active device area is assumed to be a square and the coherence length is 0.75 times the square side. This follows because researchers are generally limited to large graphene sheets, which prevents researchers from fully accounting for edge effects. Therefore, unless otherwise stated, the minimum-size transistor in the examples below has been conservatively selected to be 100 nm-by-100 nm, though considerations for scaling to smaller dimensions are also disclosed below. 
     Yet further, those having ordinary skill in the art will recognize that the aforementioned design parameters are merely examples and that the characteristics of the disclosed devices may vary. For instance, in some examples the disclosed devices may be asymmetric with no BG, even though N-type and P-type transistors may be required. In other words, to achieve the same functionalities, a SymFET with a constant V BG  can be replaced with a device without a BG but with a doped drain (or source). 
     Inverter/Buffer 
     Turning now to  FIG. 3A , an example inverter circuit  300  having a first SymFET T 1  and a second SymFET T 2  is shown. If a bias voltage (V B ) is set close to half of a positive supply voltage (V DD ) of the second SymFET T 2  (i.e., V DD /2), then for an input voltage (V in ) close to zero the first SymFET T 1  is off because a top gate voltage (V TG ) minus a back gate voltage (V BG ) differential is negative (V TG −V BG &lt;0), and the second SymFET T 2  is on. As a result, an output voltage (V out ) will be close to the positive supply voltage V DD  of the second SymFET T 2 . As shown in a voltage-time chart  302  in  FIG. 3B , a high-level input leads to a low-level output.  FIG. 3B  illustrates the relationship between input and output signals of the example inverter circuit  300  when loaded by a fanout-of-4 (FO4) inverter at V DD =0.3 V. Furthermore,  FIGS. 4A-4C  illustrate how stable quiescent operating points  400  and V out  of the example inverter circuit  300  can be determined using a characteristic of transistors where V DD  is small. In particular,  FIGS. 4A-4C  show the respective currents through T 1  and T 2  at various input voltages V in  when V DD =0.3 V.  FIG. 4D  shows a chart  402  illustrating the transfer characteristics of the example inverter  300  when V DD =0.3 V. The operation of the example inverter circuit  300  is similar to that of a CMOS inverter. Notably, the current in the off transistor is not zero, and consequently the output levels will not be equal to ground and V DD . This is no problem so long as the noise margin is sufficiently large, namely, where V DD  is greater than 0.2 V. Still, the leakage current results in static power dissipation. 
       FIGS. 5A-5D  are similar to  FIGS. 4A-4D , but show stable quiescent operating points  500  of the example inverter circuit  300  when V DD  is somewhat higher at 0.5 V.  FIGS. 5A-5C  show the respective currents through T 1  and T 2  at various input voltages V in  when V DD =0.5 V. When V in =V DD /2, the example inverter circuit  300  becomes bistable due to NDR. More specifically, when the input signal V in  is close to V DD /2, the circuit  300  has two possible outputs for the same input when V DD =0.5 V. This condition is also shown in a chart  502  plotting V out  against V in  where V DD =0.5 V.  FIG. 5C , moreover, also shows a third possible, but unstable, point where I DS1 =I DS2  when V in =0.6 V DD . This point is unstable because it lies on the NDR region of the I-V curve. The bistability manifests itself as the hysteresis in the characteristic of the gate, resulting in a Schmitt-trigger inverter. For example, when output is high, and V in  is increasing, the output remains high even for an input close to V DD /2. But when input approaches roughly 0.6 V DD , the circuit  300  changes from bistable to monostable and the output V out  toggles to a low level, as shown in the chart  502 . 
     In some instances, a low-voltage Schmitt-trigger inverter may be required. Therefore, in some examples, a positive feedback  600  from a BG  602  to a drain  604  can be incorporated into an example Schmitt-trigger inverter circuit  606  as shown in  FIG. 6A . As shown in an I out −V out  curve  608  of  FIG. 6B , the feedback  600  brings the current of transistors T 1  and T 2  down at large |V DS | and makes the I out −V out  curve  608  similar to that of  FIGS. 4A-4C , where Vin=V DD /2. The attained hysteresis for a small V DD  of 0.3V is observed in a chart  610  plotting V out  against V in  in  FIG. 6C . As those having ordinary skill in the art will recognize, the difference between the example circuit  300  in  FIG. 3A  and the example circuit  606  in  FIG. 6A  can be explained when the input is increasing and just passes V DD /2. For the circuit  300 , the value of V TG1 −V BG1 =V in −V DD /2 becomes positive and hence I DS1  becomes larger than I DS2 , whereas for the circuit  606 , V TG −V BG  of T 1  is still negative for V in ≈V DD /2 as the BG  602  is connected to an output node  612  coupled to V out . 
     An example buffer  700  is illustrated in  FIG. 7A . Compared to an inverter, the role of TG and BG terminals of transistors T 1  and T 2  are interchanged in the example buffer  700 . The buffer  700  can also exhibit hysteresis in its characteristic, as shown in a chart  702  plotting V out  against V in . If the hysteresis is undesirable, an example circuit  800  shown in  FIG. 8A  may be used instead. In the example circuit  800 , added negative feedback  802  eliminates the bistability evident in the example buffer  700 . A chart  804  plotting V out  against V in  demonstrates that the example circuit  800  does not involve hysteresis. Further, those having ordinary skill in the art will recognize that with static CMOS logic it is not possible to construct circuits similar to those illustrated in  FIGS. 7A and 8A . As a result, two inverters are often cascaded to build a buffer. 
     NAND/NOR/IMPLY 
     An example NAND gate  900  is shown in  FIG. 9A . Although the example NAND gate  900  provides desirable functionality, it is not any less complex than its static CMOS counterpart. That said, more compact gates are made possible by using the nonlinear behavior of SymFETs to build pseudo-SymFET gates (similar to pseudo-nMOS style logic). 
     In  FIG. 9B , an example SymFET  902  with feedback  904  is used as the load. When compared with the conventional NAND gate  900  shown in  FIG. 9A  using conventional logic, the SymFET  902  using pseudo-SymFET logic lowers the number of transistors required. For instance, if a conventional static CMOS circuit has N transistors, a pseudo-SymFET circuit will only require (N/2)+1 transistors. Further, the input capacitance of the gate is also cut by one half. The benefit of using a SymFET load compared with a linear resistor can be explained by way of an I-V curve  906  having a bell-curve shape  908  shown in  FIG. 9C . In this example, the current passing through the device will be small (close to I v1 ) for a low output level. In contrast, for a resistor, the current will be the largest when the output of the gate is low, resulting in large leakage. Similarly, in pseudo-N-type metal-oxide-semiconductor (pseudo-NMOS) circuits, the current is the largest when the output signal is low, since MOSFETs do not have NDR. 
     An example pseudo-SymFET NOR gate  910  having three SymFETs T 1 , T 2 , and T 3  is shown in  FIG. 9D . If the third SymFET T 3  is replaced with a 200 kΩ resistor, a similar output DC level and swing is achieved as depicted in  FIGS. 9E-9F . However, the average power dissipation when using SymFET and resistive loads is 0.16 μW and 0.51 μW, respectively, for the input waveform shown in  FIGS. 9E-9F . Hence, there is more significant leakage when a resistor is employed. More significant improvements with respect to power would be possible if I v1  was reduced. However, a small I v1  also has an adverse effect, namely, when the output changes from low to high, the load capacitance is initially charged with a current of ≈I v1 . This results in a slow rise time as seen in  FIGS. 9E-9F , wherein the low-to-high delay with the SymFET-based and resistor-based loads is 1.6 ns and 0.46 ns, respectively. 
     To enhance the speed of the pseudo-SymFET gates, one might think to increase the size of the load transistor to have a larger I v1 . However, this could result in an incorrect output because the pull-down network might not be able to bring the output voltage down from a high state. Instead, in some examples a clocked-supply is used, as in resistance thermometer detector (RTD)-based circuits. The resulting circuit, which is referred to herein as “dynamic pseudo-SymFET logic”  1000  and includes three SymFETs T 1 , T 2 , and T 3 , is shown in  FIG. 10 . In this example, an output signal is reset to zero during each clock cycle. The delay of the dynamic pseudo-SymFET circuit  1000  is reduced to 0.39 ns, whereas the power consumed from a clock input CLK is 0.12 μW. As shown in  FIGS. 10B-10D , if the size of the third SymFET T 3  is three times that of the first and second SymFETs T 1  and T 2 , the pseudo-SymFET NOR gate  910  of  FIG. 9D  can produce a wrong output, but the dynamic pseudo-SymFET gate  1000  functions correctly. Besides a larger third SymFET T 3 , another aspect of the dynamic pseudo-SymFET logic  1000  helps improve the rise time of the circuit. More specifically, as the clock input CLK is rising, an output node is simultaneously being charged. When the clock input CLK reaches V DD , the output voltage is somewhat higher than zero, and |V DS3 |=V R  is smaller than V DD . As a result, I DS3  will be larger than I v1  (see, e.g.,  FIG. 9C ). Subsequently, the output node will be charged by a current larger than that of  FIG. 9D . 
     The IMPLY gate is another universal gate besides NAND and NOR that may be implemented with SymFETs. To that end,  FIG. 11A  shows an example IMPLY gate  1100  implemented by way of first and second SymFETs T 1  and T 2 , whereas  FIG. 11B  shows an example IMPLY gate  1102  based on CMOS elements. In both circuits, an output F can be low only when A is high and B is low, and therefore the first SymFET T 1  is on, as reflected in a truth table  1104  of  FIG. 11C . However, unlike the SymFET-based IMPLY gate  1100 , the CMOS-based IMPLY gate  1102  has a shortcoming that often precludes its usage. Specifically, when A is high and B transitions from low to high, the charging of the output capacitive load is performed via a CMOS transistor T 1  in  FIG. 11B . Moreover, as F increases and approaches V DD −V TH  (i.e., where V TH  is the threshold voltage), the CMOS transistor T 1  is almost turned off, causing incomplete settling. By contrast, the SymFET-based IMPLY gate  1100  of  FIG. 11A  does not have this problem because it does not have a large V TH . This much is demonstrated in  FIGS. 11D-11F , where the functionality of the SymFET-based IMPLY gate  1100  at a V DD  of 0.3 V, and 130-nm CMOS-based IMPLY gate  1102  at a V DD  of 1 V are examined. In short, the SymFET-based IMPLY gate  1100  does not have this disadvantage inherent with the CMOS-based IMPLY gate  1102 . One advantage of the example SymFET-based IMPLY gate  1100  is its fewer transistors in comparison with NAND or NOR gates. Traditional realization of IMPLY gates requires an inverter and a NAND gate, amounting to six transistors. For instance, two XOR gates  1200 ,  1202  are shown in  FIGS. 12A-12B . The realization using the NAND-based design in  FIG. 12A  has sixteen transistors, whereas the IMPLY-based design in  FIG. 12B  has eight transistors. 
     MAJORITY Gate 
     Another useful gate that is popular at least in the context of cellular automata and spintronics is the MAJORITY gate. It is possible to construct a SymFET-based MAJORITY gate with as few as three devices. The starting point for the SymFET-based MAJORITY gate is an example resistive network  1300  shown in  FIG. 13A . If V in1 =V in2 =0 V and V in3 =V DD  in the example resistive network  1300 , the output voltage V out  will be as follows:
 
 V   out   =V   DD ( R   1   ∥R   2 )/[( R   1   ∥R   2 )+ R   3   ]=V   DD /3
 
     To convert the circuit into a MAJORITY gate for this input combination, the output voltage level V out  should be brought close to zero. The voltage across R 1  and R 2  is smaller than that of R 3 , and if resistors are replaced with nonlinear elements whose resistances increase with voltage, R 3  will become larger than R 1  and R 2  and the output V out  will be pushed toward ground. An example SymFET-based circuit  1302  of  FIG. 13B  is designed for this purpose. If input levels are 0 and V DD , both gates of first, second, and third SymFETs T 1 , T 2 , T 3  are set to V B =V DD /2 (see the case of V TG =V BG  in  FIG. 2  for the I-V curve of the device). The transient response of the example SymFET-based circuit  1302  when loaded with an FO4 inverter is shown in  FIGS. 13D-13E . When both V inB  and V inC  are zero, an output F is also zero independent of input V inA . Moreover, V inA  changes the output F when one and only one of the V inB  or V inC  inputs is high. Here, each of the transistors T 1 , T 2 , T 3  behaves like a symmetric nonlinear resistor with an I DS −V DS  curve  1304  like that illustrated in  FIG. 13F . If V inA =V inB =V inC =0 V, the output F will be zero since there is no path to charge the output F. Similarly when all of the inputs V inA , V inB , and V inC  are set to V DD , the output voltage F will equal V DD  as well. In another example, if the inputs V inA =V inB =0 V and V inC =V DD  (i.e., 001), T 3  has a large |V DS |, and a large resistance as seen in  FIG. 13F . 
     Further,  FIG. 13G  provides still more insight as to the operation of the example SymFET-based circuit  1302 .  FIG. 13G  shows a total current  1306  discharging an output node (I SD1 +I SD2 =2×I SD1 ) and a current  1308  charging the output (I DS3 ). An intersection  1310  of the two curves  1306 ,  1308  gives the output voltage F, which is close to zero as desired. The quiescent current of the transistors T 1 , T 2 , T 3  is I SD1 =I SD2 ≈I v1 /2 and I DS3 ≈I v1 . Even though a small I v1  is ideal with respect to power dissipation considerations, the two curves should not have more than one intersection for correct operation. This means that in most examples I v1  will be greater than I pk /2. 
     An example diode-connected SymFET  1400  (e.g., where the TG and the BG are shorted, respectively, to the drain and the source) may in some instances exhibit the same behavior as shown in  FIG. 13F . This in turn eliminates the need for V inB . Even a single graphene-insulator-graphene junction without any gate exhibits this characteristic, and hence the example diode-connected SymFET  1400  of  FIG. 14  can be utilized as a MAJORITY gate. 
     Combining inverters and MAJORITY gates enables the creation of an example full adder cell  1500 , as shown in  FIG. 15 . The outputs of the MAJORITY gates are buffered to isolate the gates from each other. The example full adder cell circuit  1500  is designed to be cascadable by providing output carry (C o ) and its inverted signal ( C   o ). The example full adder cell circuit  1500  has fifteen transistors in total as opposed to twenty-eight in a conventional CMOS adder cell such as the Mirror Adder. 
     The next few topologies for sequential circuit design utilizing SymFETs can offer low transistor count and low dynamic power dissipation. 
     RS Latch 
       FIG. 16A  shows an example SymFET-based reset/set (RS) latch  1600  that includes two-transistor circuits T 1  and T 2 . Here, two SymFETs T 1  and T 2  are used like nonlinear resistors R 1  and R 2  as shown in a circuit  1602  of  FIG. 16B . If inputs A and B in  FIG. 16B  have the same logic level, an output F will have the same level, independent of whether the resistors R 1  and R 2  are linear or nonlinear. If one input (e.g., A) is low and the other one (e.g., B) is high, the circuit  1602  is bistable and, as shown in a chart  1604  in  FIG. 16C , has two stable quiescent points  1606 ,  1608 . I-V curves  1610 ,  1612  of the two transistors T 1 , T 2  have three intersections, one intersection  1614  of which is unstable because it lies on an NDR region of the I-V curves  1610 ,  1612 . As a result, the output voltage F can be either low or high when one and only one of the two inputs A, B is high, and is determined by its previous state. 
     A truth table  1616  of the example SymFET-based RS latch  1600  is represented in  FIG. 16D , where “F*” refers to the previous “F” value. The truth table  1616  of the SymFET-based RS latch  1600  resembles that of a latch. In fact, adding an inverter  1700  to the SymFET-based RS latch  1600  of  FIG. 16A  results in an RS latch  1702  as shown in  FIG. 17A  that is known in the art. A truth table  1704  corresponding to the RS latch  1702  is shown in  FIG. 17B . In addition,  FIG. 17C  shows a conventional RS latch  1706 , which has eight transistors. By way of comparison, the example SymFET-based RS latch  1600  has only four transistors. A simulated transient response  1800  of the example SymFET-based RS latch  1600  is shown in  FIGS. 18A-18B , where a 10%-to-90% rise time is 0.8 ns. 
     Latch 2 
       FIG. 19A  shows another example SymFET-based latch  1900 , with a corresponding truth table  1902  in  FIG. 19B . First and second transistors T 1 , T 2  form a core part of the example SymFET-based latch  1900 , whereas third and fourth transistors T 3 , T 4  are used primarily to speedup the example SymFET-based latch  1900 . As shown in the truth table  1902 , the SymFET-based latch  1900  has three valid input combinations, although a combination where R=0 and  S =1 is not allowed. Operation of the SymFET-based latch  1900  (without speedup) can be explained using  FIGS. 19C-19E . For example, if R= S =0, then the first transistor T 1  is off and the second transistor T 2  is on. Consequently, output capacitance is charged, and a final output voltage V Q  is close to V DD . Further, if R=1 and  S =0, the SymFET-based latch  1900  is bistable. Depending on the previous Q, the output voltage V Q  can be either low or high. 
       FIGS. 20A-20C  shows transient simulation results  2000  of the SymFET-based latch  1900  without the speedup transistors T 3 , T 4 . The transient simulation results  2000  reveal large rise and fall times. The reason behind the large rise and fall times is the small charge and discharge current. For instance, consider a case where output is initially low and then input is set to R= S =0. The output capacitance must be charged, but the charging current (i.e., the difference between the currents of T 1  and T 2  (I 2 −I 1  in  FIGS. 19C-19E )) is small for a large range of output voltages. 
     To accelerate the transient response of the SymFET-based latch  1900 , the third and fourth transistors T 3 , T 4  are employed. In some examples, the BGs of the third and fourth transistors T 3 , T 4  are connected to their respective source terminals, and therefore V TG3 −V BG3  is larger than V TG1 −V BG1  for high-level R. This shifts the peak current of an I DS −V DS  curve of the third transistor T 3  to a higher V DS . The sum currents I DS1 +I DS3  and I SD2 +I SD4  will have two peaks, as shown in  FIGS. 21A-21C . Comparing  FIGS. 19C-19E  (i.e., the I-V curve of the first and second transistors T 1  and T 2  with no speedup transistors) and  FIGS. 21A-21C  (i.e., the I-V curve of transistors where I D13 =I DS1 +I DS3  and I S24 =I SD2 +I SD4 ) shows that operation of the SymFET-based latch  1900  after adding the speedup transistors T 3 , T 4  does not change in principle. The main difference is that charge and discharge currents will be higher. In the aforementioned example, wherein an output node was supposed to be charged, adding the fourth transistor T 4  increases the charging current from I 2 −I 1  to I 24 −I 1  at certain output voltages (i.e., I 24 =I 2 +I 4 ). On average, the charge current is significantly larger. The effect of speedup transistors can also been in  FIGS. 20A-20C , where the rise time of the SymFET-based latch  1900  is reduced from 5.1 ns to 0.83 ns after adding speedup transistors. 
     Mobile DFF 
     Furthermore, some features of SymFETs resemble that of resistance temperature detectors (RTDs), and SymFETs can achieve RTD-like behavior.  FIG. 22A  shows an example circuit  2200  built by placing two SymFETs T 1 , T 2  in parallel. As such, the current passing through the circuit  2200  is the sum of the currents in the first and second SymFETs T 1 , T 2 .  FIG. 22B  shows a chart  2202  plotting input current I i  against input voltage V i . In this example, each of the transistors T 1 , T 2  contributes to peaks  2204 ,  2206  in a total current  2208  and an N-shaped curve expected from an RTD is obtained for a properly designated voltage range. Those having ordinary skill in the art will appreciate that the shape of total current curve  2208  can be tuned by adjusting the bias voltages V B1 , V B2  in the circuit  2200 . 
     That said, there are still other ways to achieve this functionality without replacing an RTD with two SymFETs. By way of example, one of the most popular RTD-based logic elements is a monostable-bistable transition logic element (MOBILE). MOBILEs have a self-latching property. For instance, a circuit  2300  shown in  FIG. 23  works both as an inverter and as an edge-triggered flip-flop (DFF). The example circuit  2300  includes a switch  2302 , implemented by a transistor, and three RTDs D 1 , D 2 , and D 3 , which should be properly sized such that their peak currents satisfy the following conditions:
 
 I   pk2   &lt;I   pk3  and  I   pk1   +I   pk2   &gt;I   pk3 .
 
     In this example, operation of the circuit  2300  relies heavily on NDR regions of the three RTDs D 1 , D 2 , and D 3 , as reflected in charts  2304 ,  2306 ,  2308  of  FIGS. 23B-23D . When a clock input CLK is a small voltage, the circuit  2300  is monostable and an output F is low. As the clock input CLK increases (e.g., from much less than V DD  to 2×V pk  to V DD  shown in  FIGS. 23B-23D ), a driving current curve (I D1 +I D2 ) will intersect with a load current curve (I D3 ) in an NDR region, resulting in metastability. When the clock input CLK becomes high enough, the output reaches one of two stable points. The example shown in  FIGS. 23B-23D  corresponds to a low A (e.g., I D1 =0) and a high final output voltage. 
     Still further, an example SymFET-based MOBILE  2400  is shown in  FIG. 24A . A first SymFET T 1  acts as both the switch A and the first RTD D 1  from  FIG. 23A . Moreover, a fourth SymFET T 4  may be omitted in some examples, but utilized in others. The fourth SymFET T4 helps to discharge the output faster when a clock input CLK switches from high to low. Proper operation of the example SymFET-based MOBILE circuit  2400  depends on sizing of the first, second, and third SymFETs T 1 , T 2 , T 3  as well as bias voltages (V B1-2 ). In some examples, there are many ways to satisfy the conditions that I pk2 &lt;I pk3  and that I pk1 +I pk2 &gt;I pk3 . In the example SymFET-based MOBILE  2400  here, the second and third SymFETs T 2 , T 3  are minimum-size transistors, while in other examples they may be larger. Nonetheless, when input is low, I DS1  is negligible. V DD −V B2  is positive (0.3 V), but the gate voltages of the second SymFET T 2  are both grounded. This insures that a peak current of the second SymFET T 2  is smaller than that of the third SymFET T 3  as desired (I pk2 &lt;I pk3 ). Further, V B1  is selected to be V IH −0.3 V so that the gate differential of T 1  and T 3  are equal, when input is high. The size of the second SymFET T 2  is twice that of the first SymFET T 1  and, therefore, a peak current of the second SymFET T 2  is higher than that of the third SymFET T 3 . This in turn insures that a high-level input will result in a low-level output.  FIGS. 24B and 24C  show current through the SymFETs of the MOBILE circuit  2400  for a high clock input CLK (e.g., equal to V DD =1 V). The SymFET-based MOBILE circuit  2400  is bistable for both input logic levels. 
     With reference now to  FIGS. 24D-24F , input/output waveforms  2402  of the example MOBILE circuit  2400  are shown. Input levels have been selected to be 0.1 V and 0.7 V, similar to worst-case output levels. Those having ordinary skill in the art will understand that altering A does not change the output, even when the clock input CLK is high. 
     Benchmarking 
     The quantitative performance of the aforementioned example SymFET-based gates was evaluated using SPICE simulations. As shown below, Table I compares the metrics of several inverters and buffers in terms of peak-to-peak output swing (V o,pp ), propagation delay (T pd ), average dynamic energy per output switching (E DYN ), and static power dissipation (P STAT ). 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 PERFORMANCE OF INVERTERS/BUFFERS 
               
             
          
           
               
                   
                 V o, pp   
                 T pd   
                 T rise , T fall   
                 E DYN   
                 P STAT   
               
               
                 Circuit 
                 (V) 
                 (ns) 
                 (ns) 
                 (fJ) 
                 (nW) 
               
               
                   
               
               
                 Inv. FIG 3A 
                 0.25 
                 0.28 
                 0.64 
                 0.13 
                  71 
               
               
                 Inv. FIG. 6A 
                 0.19 
                 0.32 
                 0.61 
                 0.1 
                  45 
               
               
                 Buf. FIG 7A 
                 0.17 
                 0.20 
                 1.3 
                 0.08 
                  68 
               
               
                 Buf. FIG. 8A 
                 0.13 
                 0.13 
                 0.85 
                 0.03 
                 120 
               
               
                 Inv. FIG. 9D* 
                 0.23 
                 0.91 
                 1.5, 0.83 
                 0.15 
                  68 
               
               
                 Inv. FIG. 9A* 
                 0.22 
                 0.29† 
                 0.7, 0.57 
                 0.12/.07‡ 
                  62 
               
               
                   
               
               
                 *without T2. 
               
               
                 †from CLK to F (from A to F for others). 
               
               
                 ‡depends on capability of CLK to absorb power (see the text). 
               
               
                 Note: 
               
               
                 all simulations are done with a FO4 inverter of FIG. 3A as driver and load of the gate under test at V DD  = 0.3 V (= high level of CLK). Rise/fall times (T rise , T fall ) are measured from 10% to 90% of the output levels. 
               
             
          
         
       
     
     Some of the example gates have a smaller output swing (and hence a smaller noise margin), which results in lower dynamic energy dissipation. As in CMOS designs, each gate might be suitable for a certain application. For example, the example inverter of  FIG. 9  has a larger delay compared with that in  FIG. 3A , but also has a smaller input capacitance. In another example, the clocked inverter of  FIG. 10A  might be viewed as an adiabatic circuit. That is, when the clocked input CLK goes low, the charge stored at the output node F will return back to the value at the clock input CLK node. Depending on whether the clock input CLK generator circuitry is capable of absorbing this charge or simply directs it to ground, two different dynamic energy values are provided in Table I. 
     In Table II, the performances of three example SymFET-based adders based on conventional topology (mirror adder), a dynamic pseudo-SymFET design, and a MAJORITY gate are compared. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 PERFORMANCE OF 1-BIT SYMFET-BASED FULL-ADDERS 
               
             
          
           
               
                   
                 T pd   
                 E DYN   
                 P STAT   
                 P TOT   
                 PDP 
                 Area* 
               
               
                   
                 (ns) 
                 (fJ) 
                 (μW) 
                 (μW) 
                 (fJ) 
                 (μm 2 ) 
               
               
                   
               
               
                 Conventional (0.3 V) 
                 1.4  
                 0.4  
                 0.64 
                 0.72 
                 1.0  
                 0.64 
               
               
                 Dyn. pseudo-Sym. (0.4 V) 
                 1.1  
                 0.95 
                 0.61 
                 0.8  
                 0.88 
                 0.49 
               
               
                 Majority (0.4 V) 
                 0.65 
                 0.37 
                 1.1  
                 1.2  
                 0.78 
                 0.42 
               
               
                   
               
               
                 *assuming total area (including wirings) is twice the area of transistors. 
               
               
                 Note: 
               
               
                 In SPICE simulation, a 100 MHz signal is applied to A while B is Low and C i  is High. 
               
               
                 The adder is cascaded with a similar one. 
               
             
          
         
       
     
     For the purposes of Table II, a slightly higher supply voltage is used for the MAJORITY-based adder in order to have the same output swing in both circuits. The MAJORITY-based adder has a better power-delay product (PDP) and occupies less area, but has higher static power dissipation. Those having ordinary skill in the art will recognize that at this test frequency (i.e., 100 MHz), the power dissipation is dominated by leakage. 
     Likewise, it is informative to benchmark the performance of SymFET technologies against CMOS technologies. Due to the different structure of the SymFET, it is non-trivial to select a certain CMOS technology node for a comparison. In the examples that follow, a 100-nm SymFET device is compared with 90-nm and 130-nm CMOS technologies. According to a manual of several standard cell libraries for said technologies, the dynamic energy of the minimum-size inverter and full-adder cell are, respectively, 4 to 6 fJ and 10 to 20 fJ. The dynamic energy of a CMOS DFF is 20 to 30 fJ. Compared with the E DYN  in Tables I, II, and IV, the SymFET-based circuits are approximately one order of magnitude more energy efficient. This is especially impressive for the MOBILE DFF, which has a delay close to those given for standard cell DFFs (around 0.1 ns). However, in some examples, the leakage of CMOS standard cells is smaller (in sub-nW range for low-leakage CMOS). 
     Moreover, an inverter of  FIG. 3A  was used as a basis for further comparison. The results shown in  FIG. 25  involve a 100-nm SymFET device and a 130 nm CMOS device. All results belong to a minimum-size inverter that is both driven and loaded with FO4 inverters. More particularly,  FIG. 25A  shows power dissipation of a SymFET-based inverter. The dynamic power dissipation rapidly changes with supply (cf., CV 2 ). In this example, a relationship appears between propagation delay of the SymFET-based gate and V DD , shown in  FIG. 25B , which is quite different than the behavior of CMOS inverters. An explanation follows with reference back to  FIG. 2 . If V DD  (and consequently the gate voltage of the transistor) is large, the peak in the I DS  curve is pushed to higher V DS  values. This means that for a large V DD , the current drive capability of the SymFET-based inverter is relatively small in a large output voltage range and the gate becomes slow.  FIG. 25C  shows both delay and energy at different supply voltages. In this example, the SymFET has a better performance-energy tradeoff, if the desired delay is not demanding. 
     Next, the performance of the inverters is compared at several technology nodes. The data for CMOS is from A. Stillmaker et al., “Toward more accurate scaling estimates of CMOS circuits from 180 nm to 22 nm,” Technical Report ECE VCL 2011 4 VLSI Computation Lab, Univ. of Cali., Davis, http://www.ece.ucdavis.edu/vcl/pubs, which is hereby incorporated by reference in its entirety. The SymFET dimensions and other properties are given in Table III, wherein EOT is also reduced with device dimension to enhance the electrostatic control of the gates. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 SYMFET SCALING 
               
             
          
           
               
                 Dimensions 
                 Gate Oxide 
                 V DD   
                 Inverter delay 
               
               
                   
               
               
                 100 nm-by-100 nm 
                 EOT = 1.2 nm 
                  0.3 V 
                 0.28 ns 
               
               
                  50 nm-by-50 nm 
                 EOT = 1.0 nm 
                 0.36 V 
                 0.49 ns 
               
               
                  20 nm-by-20 nm 
                 EOT = 0.7 nm 
                 0.49 V 
                  1.2 ns 
               
               
                   
               
             
          
         
       
     
     As the SymFET area is reduced, its characteristic changes somewhat (off-current increases). To keep the voltage swings in a practical range (i.e., &gt;0.25 V), the supply voltage (Table III) was increased. When compared with the CMOS device in  FIG. 25D , the SymFET exhibits more than an order of magnitude improvement in dynamic energy. One having ordinary skill in the art will recognize that if operation frequency increases, the impact of leakage in total energy will be reduced and “Total” energy points will move toward “Dynamic” energy points. However, it is also possible to reduce the dynamic energy dissipation of CMOS gates by an order of magnitude by lowering VDD by a factor of 3.1, where the resulting VDD of 0.3 to 0.4 V would make their delay larger than that of the SymFET gates (see  FIG. 25C ). Consequently, SymFETs prove more energy efficient than CMOS at least when clock frequencies of a few hundred MHz and delays on the order of few tenths of nanoseconds are desired. 
     What&#39;s more,  FIG. 25D  indicates that dynamic energy of the SymFET scales rapidly with device dimension. When the SymFET is scaled by λ (&lt;1), its area and hence both capacitance and current drive are reduced by two. Energy and delay are scaled as 
     
       
         
           
             
               
                 E 
                 DYN 
               
               ⁡ 
               
                 ( 
                 
                   α 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     C 
                     L 
                   
                   ⁢ 
                   
                     V 
                     SW 
                   
                   ⁢ 
                   
                     V 
                     DD 
                   
                 
                 ) 
               
             
             ⁢ 
             
               → 
               scaled 
             
             ⁢ 
             
               
                 λ 
                 2 
               
               ⁢ 
               
                 E 
                 DYN 
               
             
           
         
       
     
                 T   pd     ⁡     (     α   ⁢         C   L     ⁢     V   SW           I   ON     -     I   OFF           )       ⁢     →   scaled     ⁢     T   pd           
where C L  is the load capacitance, V SW  is the output swing, and I ON  and I OFF  are the on-current and off-current of the transistor. The above equation suggests that delay will not be scaled with technology. Table III shows that simulated T ps  increases with scaling. This is a consequence of higher gate capacitance, which is inversely proportional to EOT, and also lower I ON −I OFF  attributable to the higher off-current.
 
     Yet another tool can be used to modify the performance of the SymFETs and trade off delay with leakage, namely, the thickness of the tunneling barrier. In some examples, 1.34 nm BN was used as discussed in L. Britnell et al., “Field-effect tunneling transistor based on vertical graphene heterostructures,” Science, vol. 335, no. 6071, pp. 947-950, February 2012, which is hereby incorporate by reference in its entirety. Further, if fabrication technology and breakdown voltage of the insulator allow for a thinner barrier, tunneling current will substantially increase. For instance, if barrier thickness is reduced to 0.67 nm (i.e., 2 layers of BN), then delay of the inverter will be reduced by at least 10,000 times, even though the leakage current may increase proportionally. 
     Similar to Tables I and II above, the performance of SymFET latch and MOBILE DFF devices were also evaluated, as shown in Table IV below. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                 PEFORMANCE OF INVERTERS/BUFFERS 
               
             
          
           
               
                   
                 V DD   
                 V o, pp   
                 T pd   
                 E DYN   
                 P STAT   
               
               
                   
                 (V) 
                 (V) 
                 (ns) 
                 (fJ) 
                 (nW) 
               
               
                   
               
               
                 RS Latch FIG. 17A 
                 0.5 
                 &gt;0.4 
                 0.41 
                 0.5 
                 270 
               
               
                 Latch FIG. 19A 
                 1 
                   0.84 
                 1.8 
                 2 
                 640 
               
               
                 Inv./DFF FIG. 24A 
                 1* 
                   0.71 
                 0.06 
                 2.9/0.71† 
                 480 
               
               
                   
               
               
                 *high level of CLK. 
               
               
                 †depends on capability of CLK to absorb power (see the text). 
               
               
                 Note: 
               
               
                 all simulations are done with a FO4 inverter of FIG. 3A as load. 
               
             
          
         
       
     
     Therefore, several new topologies for circuit design involving SymFETs have proven that they can offer low transistor count and low dynamic power dissipation. More specifically, SymFET-based inverters provide a lower switching energy compared with the iso-delay CMOS-based circuits, due in large part to their low operating supply voltage. 
     Although certain example devices have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all devices, methods, apparatus, systems, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Technology Classification (CPC): 7