Abstract:
A logic gate including a resonant-tunneling transistor and a resistor connected in series thereto. The resonant-tunneling transistor has a superlattice structure. The resonant-tunneling transistor may be a resonant-tunneling hot electron transistor or a resonant-tunneling bipolar transistor. The resonant-tunneling transistor conducts a current between a collector and an emitter. The current has one of at least three different current values in response to a base voltage of one of three different voltage values. The third current value is between the first and second current values, and a second voltage value is between the first and third voltage values. The logic gate outputs one of at least three states, a high state, a low state and a state approximately between the high and low states in response to a signal applied to the logic gate. The signal has an amplitude of one of the first to third voltage values. A logic circuit includes at least three connected resonant-tunneling transistors. The logic circuit maintains at least three states, a high state, a low state, and a state approximately between the high and low states in the respective three resonant-tunneling transistors in response to a pulse signal applied to a base of one of the resonant-tunneling transistors.

Description:
&#34;This is a continuation of co-pending application Ser. No. 917,060 filed on Oct. 9, 1986 now abandoned.&#34; 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ternary (tristable) logic circuit having resonant-tunneling transistors, such as resonant-tunneling hot electron transistors (RHETs) and resonant-tunneling bipolar transistors (RBTs). More particularly, it relates to a tristable gate circuit, a latch circuit having tristates, and a memory cell having tristates. 
     2. Description of the Related Art 
     Binary logic circuits have been extensively used and the fact that logic circuits of e (a base of a natural logarithm, e=2.718), are ideal has been proved in theory. Theoretically, the performance of a ternary logic circuit is superior to that of a binary logic circuit, and therefore, attempts have been made to realize ternary logic circuits. The prior art ternary logic circuits are not used in practice, because of small margin, complex circuit arrangement, etc. 
     Conversely, the principle of a resonant-tunneling transistor, such as a RHET has long been known. Recently, due to advances in semiconductor processing technology, such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), practical RHETs have been developed (e.g., &#34;RESONANT-TUNNELING HOT ELECTRON TRANSISTORS (RHET): POTENTIAL AND APPLICATIONS&#34;, N. Yokoyama, et al, Japanese Journal of Applied Physics, Vol. 24, No. 11, November, 1985, pp. L853-L854). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a ternary logic circuit with a simple circuit arrangement and stable operation, by using resonant-tunneling transistors. 
     According to the present invention, there is provided a logic gate including a resonant-tunneling transistor and a resistor connected in series thereto. The resonant-tunneling transistor operates such that a current flows between a collector and an emitter, and this one of at least three different current values, i.e., a first, a second, or a third value, in response to a base voltage having one of these different voltage values, i.e., a first, a second or a third value. The third current value lies between the first and second current values. The second voltage value lies between the first and third voltage values. The logic gate outputs one of at least three states having a high value, a low value, and a value approximately between the high and low values in response to a signal applied to the logic gate. The signal has an amplitude corresponding to one of the first to third voltage values. 
     The resonant-tunneling transistor may be a resonant-tunneling hot electron transistor or a resonant-tunneling bipolar transistor. 
     The second voltage value applied to the base may be approximately 2·E X  /q, where E X  is an energy level of a sub-band at a quantum well in a superlattice in the resonant-tunneling transistor, and q is the charge of carriers in the resonant-tunneling transistor. The first voltage may be lower than 2·E X  /q. The third voltage may be higher than 2·E X  /q. 
     The quantum well of the superlattice is formed between first and second barrier layers. 
     In addition, a logic gate according to the present invention can be provided where a resonant-tunneling transistor, connected in series with a resistor, receives an input signal of the logic gate at its base for conducting a current between a collector and an emitter through a superlattice layer. The input signal is one of three voltage ranges. The value of the current depends on the voltage value of the input signal. The current characteristic with respect to the input voltage value has a minimum current value, a local maximum current value at an input voltage corresponding to the resonant energy level, a local minimum current value, a first positive resistance characteristic between the minimum current value and the local maximum current value, a negative resistance characteristic between the local maximum current value and the local minimum current value, and a second positive resistance characteristic above the local minimum current value. The input voltage corresponding to the minimum current value is in the first voltage range, the input voltage corresponding to the local maximum current value is in the second voltage range, and the output voltage corresponding to the local minimum current value is in the third voltage range. The logic gate outputs an output signal having an output voltage within one of first, second, and third ranges which correspond respectively to the first, third and second input voltage ranges. 
     The input voltage value providing the local maximum current value is equal to approximately 2·E X  /q. 
     The potential height of at least one of the first and second barrier layers is arranged so that the first, second and third output voltage values are separate from each other. 
     According to the present invention, there is also provided a logic circuit including at least three resonant-tunneling transistors connected in series and forming a closed loop. 
     The logic circuit has at least three states having a high value, a low value, and a value approximately between said high and low values, in three respective resonant-tunneling transistors in response to a pulse signal applied to a base of one of the resonant-tunneling transistors. The pulse signal has an amplitude corresponding to one of first to third voltage values. 
     According to the present invention, there is further provided a semiconductor memory device including a plurality of memory cells connected between a plurality of word lines and a plurality of bit lines in a matrix fashion. Each memory cell includes an a single gate and two series-connected gates. Both common connection points are operatively connected to the word lines and the bit lines through transfer gates. Each gate includes a resonant-tunneling transistor and a resistor connected in series with the resonant tunneling transistor. Each memory cell holds at least three states having a high value, a low value and a value approximately between the high and low values in response to a signal applied between the bit lines and word lines. The signal has an amplitude corresponding to one of the first to third voltage values. 
     The semiconductor memory device may further include a plurality of sense amplifiers. Each amplifier includes a single gate and two series-connected gates. Both common connection points are operatively connectable to the bit lines to receive data from the memory cell. Each gate includes a resonant-tunneling transistor and a connected in series with the resonant tunneling transistor. 
     A semiconductor memory device may further include a plurality of write amplifiers. Each amplifier includes a single gate and two series-connected gates. Both common connection points are operatively connectable to the bit lines to output data to be stored in the memory cells. Each gate includes a resonant-tunneling transistor and a resistor connected in series with the resonant tunneling transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will be described below in detail with reference to the accompanying drawings, in which: 
     Figs 1a and 1b are views of a structure and an energy state of a resonant-tunneling hot electron transistor (RHET) respectively; 
     FIG. 1c is a view of an energy state of a resonant-tunneling bipolar transistor (RBT); 
     FIGS. 2a to 2c are graphs of an energy state of the resonant-tunneling transistor in FIG. 1a; 
     FIG. 3 is a graph of the characteristics of the resonant-tunneling transistor FIG. 1a; 
     FIG. 4 is a circuit diagram of a first embodiment of a basic logic circuit according to the present invention; 
     FIG. 5 is a graph of the operation of the logic circuit in FIG. 4; 
     FIG. 6a is a circuit diagram of a second embodiment according to the present invention; 
     FIG. 6b is a simplified circuit diagram of FIG. 6a; and 
     FIG. 6c is a truth table for the circuit in FIGS. 6a and 6b; 
     FIG. 7 is a circuit diagram of a third embodiment of a semiconductor memory device according to the present invention; 
     FIG. 8 is a graph of the operation of the semiconductor memory device FIG. 7; and 
     FIGS. 9 and 10 are modifications of the circuit in FIG. 8. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing the preferred embodiments of the present invention, a description will be given of the principle of a resonant-tunneling transistor device. 
     FIG. 1a is a sectional view of a semifinished RHET device, and FIG. 1b is a graph of an energy band of the RHET device in FIG. 1a. In FIG. 1a, the resonant-tunneling transistor device consists of a collector electrode 8, an n +  -type GaAs collector layer 1, formed on the collector electrode 8, a non-doped impurity A1 y  Ga 1-y  As (e.g. y=0.3) collector side potential barrier layer 2 formed on the collector layer 1, an n +  -type GaAs base layer 3 formed on the potential barrier layer 2, a superlattice layer 4, an n +  -type GaAs emitter layer 5, an emitter electrode 6, and a base electrode 7. The superlattice layer 4 consists of a A1 x  Ga 1-x  As barrier layer 4A 1 , a non-doped impurity GaAs quantum well layer 4B, and an A1 x  Ga 1-x  As barrier layer 4A 2 . The superlattice layer 4 functions as an emitter side potential barrier. In this specification, the superlattice is defined such that at least one quantum well is provided therein. In FIG. 1a, a plurality of quantum wells may be formed. 
     In FIG. 1b, reference EC represents a bottom of a conduction-energy band, and EX an energy level of a sub-band of the quantum well. 
     Referring to FIGS. 2a to 2c, the principle of the operation of the resonant-tunneling transistor device will be described. 
     FIG. 2a is a graph of an energy band of the RHET device when a voltage VBE between the base layer 3 and the emitter layer 5 is lower than 2·E X  /q, wherein q represents the charge of the carriers, or is too low, for example, approximately zero volt. In FIG. 2a, although a voltage V CE  exists between the collector layer 1 and the emitter layer 3, electrons at the emitter layer 5 cannot reach the base layer 3 by tunneling through the superlattice layer 4, since the base-emitter voltage V BE  is approximately zero, and thus an energy level E FE , that is a quasi-Fermi level, of the emitter layer 5 differs from the energy level E X  at the sub-band. Accordingly, a current does not flow between the emitter layer 5 and the collector layer 1. Reference φ C  represents a conduction-band discontinuity. 
     FIG. 2b is a graph of an energy band of the RHET device when the base-emitter voltage V BE  is approximately equal to 2·E X  /q. In FIG. 2b, the energy level E FE  at the emitter layer 5 is substantially equal to the energy level E X  of the sub-band at the quantum well layer 4B. As a result, due to a resonant-tunneling effect, electrons at the emitter layer 5 are passed through the superlattice layer 4 and injected into the base layer 3. The potential energy of the injected electrons, for example, 0.3 eV, is converted to kinetic energy, the electrons being in a &#34;hot&#34; state. The hot electrons are ballistically passed through the base layer 3 and into the collector layer 1. As a result, a current flows between the emitter layer 5 and the collector layer 1. 
     FIG. 2c is a graph of an energy band of the RHET device when the base-emitter voltage V BE  is higher than 2·E X  /q. In FIG. 2c, the energy level E FE  at the emitter layer 5 is higher than the energy level E X  of the sub-band at the quantum well layer 4B. The resonant-tunneling effect does not occur, and the electrons introduced from the emitter layer 5 to the base layer 3 do not exist. Consequently, the current flowing into the RHET device is reduced. On the other hand, by decreasing the barrier height of the barrier layer 4A 1  which is adjacent to the base layer 3 to a suitable value, the electrons may directly tunnel through the barrier layer 4A 2 , which is adjacent to the emitter layer 5. As a result, a certain amount of collector current may flow. 
     FIG. 1c is a graph of an energy band of a resonant-tunneling bipolar transistor (RBT). The RBT consists of an emitter layer of n +  -type GaAs, a base layer of p +  -type GaAs, and a collector layer of n +  -type GaAs. The emitter layer includes a superlattice having at least one quantum well with a sub-band energy Ex. The base layer and the collector layer are PN-joined. The RBT also exhibits a resonant-tunneling effect and the principle of operation thereof is similar to that of the RHET, and thus, is omitted. 
     FIG. 3 is a graph of the characteristics of the RHET device set forth above. In FIG. 3, the abscissa indicates the base-emitter voltage V BE  and the ordinate indicates the collector current I C . Curves C 1  to C 4  represent the characteristics when the collector-emitter voltage V CE  is respectively 2.5 V, 2.0 V, 1.5 V, and 1.0 V. 
     The curves indicate n-shaped differential negative-resistance characteristics. The present invention uses this feature to realize ternary logic circuits. 
     Referring to FIGS. 4 and 5, a basic ternary logic circuit according to the present invention and the operation thereof will be described. 
     The logic circuit 1 include a RHET 11, a signal source 12 supplying an input voltage V IN , i.e., base-emitter voltage V BE , to a base of the RHET 11, and a resistor 13 connected to a collector of the resonant-tunneling transistor 11. An emitter of the RHET 11 is grounded. Another end of the resistor 13 is connected to a DC power supply having a volt V CC . The signal source 12 outputs the voltage V IN  having a value of V INH  which is higher than 2·E X  /q, V INM  which is approximately equal to 2·E X  /q, or V INL  which is lower than 2·E X  /q. Referring back to FIG. 3, preferably a minimum value of the collector current I C  at a differential negative resistance region is half of a maximum value of the collector current due to the resonant-tunneling effect. 
     When the voltage V IN  having a low value V INL  is supplied to the base of the RHET, an output voltage V OUT  at the collector of the RHET 11 is a high level V H . When median voltage V INM  is supplied to the base of the RHET, the output voltage V OUT  is a low level V L  ; when the high voltage V INL  is supplied to the base of the RHET, the output voltage V OUT  is a median level V M . The truth table of the ternary logic circuit in FIG. 4 is as follows: 
     
                       TABLE l______________________________________   V.sub.IN     V.sub.OUT______________________________________   Low          High   Median       Low   High         Median______________________________________ 
    
     Referring to FIGS. 6a to 6b, a latch circuit using RHETs will be described. 
     In FIG. 6a, the latch circuit includes three series connected RHETs 11A, 11B, and 11C which form a loop, and three resistors 13A, 13B, and 13C. The signal source 12 is connected to the RHET lA. FIG. 6b is a simplified view of the circuit in FIG. 6a. 
     When the voltage VIN having the low level V INL  (V INL  =0) is supplied to the RHET 11A, voltages V OUT1  having a high level, V OUT2  having a middle level and V OUT3  having a low level are respectively output as shown in FIG. 6c. If the voltage V IN  is changed to median level, a low level voltage V OUT1 , a high level voltage V OUT2  and a median level voltage V OUT3  are output. When the voltage V IN  is high level, the voltages V OUT1 , V OUT2 , and V OUT3  are median, low and high level, respectively. If the voltage V IN  is changed to zero volts, the above voltage status is not changed. Then a pulse voltage having an amplitude of V INL , V INM  or V INH  is supplied to the resonant-tunneling transistor 11A, and the RHETs 11A to 11C hold one of the conditions shown in FIG. 6c in response to the amplitude of the pulse voltage. As can be easily seen, the circuit in FIG. 6a functions as a tristable latch circuit. 
     Referring to FIG. 7, still another embodiment will be described. FIG. 7 is a partial circuit diagram of a semiconductor memory device in which the RHETs are provided. 
     The semiconductor memory device includes a row decoder 21 connected to a word line WL, a column decoder 22 activating column gate transistors 23a and 23b connected between bit lines BL and BLand data buses DB and DB, and a memory cell 24 connected between the word line WL and the bit line BL, and between the word line WL and the bit line BL. The memory cell 24 consists of a latch circuit 241, as shown in FIGS. 6a and 6b, and transfer gates 242 and 243. The memory device also includes a sense amplifier 25 having three RHETs 251 to 253, a write amplifier 27 having three RHETs 271 to 273, and a pair of gate transistors 26 connected between the data buses DB and DBand the write amplifier 27. The memory device further includes a data input buffer 28, a data output buffer 29, and a clock generator 30. 
     Referring to FIG. 8, the operation of the memory device will be described. In FIG. 8, the abscissa represents a time, and the ordinate represents a signal voltage. References H, M, and L represent high level, middle level, and low level respectively of the resonant-tunneling transistor. 
     First, the read operation will be described. The read operation is carried out during times 0 to t 1 . FIG. 8 represents a condition where the read operation was carried out for a memory cell (not shown) and the voltage equalization between the bit lines BL and BLwas not fully completed, and therefore, a small voltage difference between the bit lines BL and BLstill remains. 
     Upon receipt of an address signal, from an address buffer (not shown) that has received the address signal, the row decoder 21, the column decoder 22, the word line WL, and the column gate transistors 23a and 23b are selected, as shown in FIG. 8. At a time t 1 , a column gate signal CL is raised to turn ON the column gate transistors 23a and 23b. The gates 242 and 243 are also turned ON. Voltage from the latch circuit 241 is transferred to the bit lines BL and BL. The voltages on the bit lines BL and BLare high level, middle level, or low level. The voltages are further transferred to the sense amplifier 25 through the column gate transistors 23a and 23b and the data buses DB and DB. At the time t 2 , upon receipt of a sense strobe signal φs, the sense amplifier 25 having a tri-stable flip-flop arrangement is activated to amplify the differential received voltage at a predetermined level. An output voltage amplified at the sense amplifier 25 is translated into a signal having a low impedance at the data output buffer 29, and output as an out data D OUT  to an outside of a chip of the memory device by applying an output enable signal OE. The data output buffer 29 is a quad-state buffer having a fourth high impedance (Hi-Z) state in addition to the high, median, and low states of the conventional tristate buffer. 
     Next, the write operation will be described. Upon receipt of an external inverted write enable signal WEat the clock generator 30 at a time t 4 , the column gate transistors 23a and 23b are turned ON at a time t 5 . An input data O IN  is supplied to the write amplifier 27, having a tristable flip flop arrangement, through the data input buffer 28 in response to an internal write enable signal WE1. The sense amplifier 27 has a larger drive capacity than the sense amplifier 25, but has a similar circuit construction. The amplified input data is transferred to the memory cell 24 through the gate transistors 26, the data buses DB and DB, the column gate transistors 23a and 23b, and the bit lines BL and BL. The input data is stored in the latch circuit 241 through the transfer gates 242 and 243. The content in the latch circuit 241 is one of three states. 
     Referring to FIG. 9, a modified circuit of a single bit line BL and a single data bus DB is shown. This circuit increases the integration of IC devices, but has a lower operation speed. 
     Referring to FIG. 10, another modified circuit having three bits lines BL and three data buses DB is shown. This circuit improves the operation speed, but has a lower integration. 
     In the above embodiment, the tristate data input and output circuits are used. These can be changed by a tristate-to-bistate conversion circuit to adapt to conventional binary circuits. The conversion circuit is provided in a chip. The ternary signal processing is effected in the chip, and the conventional binary signal processing is effected outside of the chip. This circuit construction has a disadvantage in that a conversion time is required. The capacity of the memory is only 1.5 times that of a binary circuit, for example, a 64K cell array corresponds to a 96K cell array. That is, the advantage of a 2 n  memory capacity cannot be obtained, and the memory addressing would be complex. Although it is inconvenient to provide the conversion circuit, an error checking and correction (ECC) circuit may be formed, because, for example, 64K cells substantially function as 96K cells. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification, except as defined in the appended claims.