MULTIPLE-VALUE LOGIC CIRCUITRY

Abstract: Logic circuitry (1) includes a resonant-tunneling transistor (11) and a resistor (13) connected in series thereto. The resonant-tunneling transistor has a superlattice structure and may be a resonant-tunneling hot electron transistor or a resonant-tunneling bipolar transistor. The resonant-tunneling transistor is operable to flow a current, between a collector and an emitter, having one of at least three different current values of a first, a second or a third value in response to a base voltage (V IN ) in one of three different voltage values of a first, second or a third value. The third current value lies between the first and second current values, and the second voltage value lies between the first and third voltage values. The logic circuitry outputs one of at least three states having a high value, a low value, and a value approximately in between the high and low values in response to a signal applied to the logic circuitry the signal having an amplitude of one of the first to third voltage values.

Description: The present invention relates to logic circuitry. Binary logic circuits have been used extensively for a long time. Theoretically, however, the performance of a ternary (tristable) logic circuit could be superior to that of a binary logic circuit, and therefore attempts have been made to realize ternary logic circuits. However, ternary logic circuits were hitherto characterised by complex circuitry and small voltage/current differences between the three logic states, which resulted in those circuits being difficult to use in practice. Advances in semiconductor processing technology, such as molecular beam epitaxy (MBE) and metal organic chemical vapour deposition (MOCVD), have enabled practical resonant-tunnelling transistors such as resonant-tunnelling hot electron transistors (RHET), to be developed (e.g., "RESONANT-TUNNELLING HOT ELECTRON TRANSISTORS (RHET): POTENTIAL AND APPLICATIONS", N. Yokoyama, et al, Japanese Journal of Applied Physics, Vol. 24, No. 11, November, 1985, pp. L853 - L854). As indicated in that document, however, logic circuitry employing known practical resonant-tunnelling transistors can provide only a total of two discrete output values in response to three discrete input signals, and hence cannot be used to form ternary logic circuits. This is because the collector-emitter current flowing in such known transistors upon application thereto of a "low" voltage is barely distinguishable from that flowing upon application of a "high" voltage. In contrast, the applicants herein propose a logic gate employing an advantageous resonant-tunnelling transistor which can provide three discrete output values in response respectively to three discrete input signals, this being achieved by forming the superlattice structure of the transistor in such a way that the potential barrier heights thereof differ from one another sufficiently to ensure that the value of the current flowing between the collector and the emitter of the transistor when the input signal voltage is "high" differs considerably from that flowing when the input signal voltage is "low". According to a first aspect of the present invention, there is provided a logic gate comprising: a resonant-tunnelling transistor having a base, an emitter and a collector, there being between the base and the emitter a superlattice structure providing a quantum well layer between first and second barrier layers, said quantum well layer having a resonant-state energy level; and a resistor connected in series, between first and second power supply lines, with the emitter-collector current path of said resonant-tunnelling transistor; the base of said resonant-tunnelling transistor serving as an input terminal of the logic gate for the application thereto of an input voltage signal constituting a base-to-emitter voltage of the said transistor when the gate is in use; and a circuit node between the said resistor and the transistor serving as an output terminal of the logic gate for delivering an output voltage signal when the gate is in use; whereby when the input voltage signal is increased, from a first working voltage value to a second working voltage value, current passing along the said emitter-collector current path is thereby caused to increase from a first current value to a second current value, and when the input voltage signal is increased from the second working voltage value to a third working voltage value such current is thereby caused to decrease from said second current value to a third current value; the said superlattice structure being so formed that when the said input voltage signal has the said third working voltage value the potential height of one of said first and second barrier layers differs from that of the other sufficiently to ensure that said third current value lies between and is distinct from the said first and second current values, so that the said output voltage signal has discrete first, second and third output values respectively when the said input voltage signal has the said first, second and third working voltage values, the said third output value of the output voltage signal being between the said first and second output values thereof. The resonant-tunnelling transistor of such a logic gate may be a resonant-tunnelling hot electron transistor or a resonant-tunnelling bipolar transistor (RBT). According to a second aspect of the present invention, there is provided latching circuitry comprising three logic gates, each according to the first aspect of the present invention, the output terminal of each gate being connected to the input terminal of another of the gates such that the gates form a closed loop; said latching circuitry thereby being operable, when a pulse signal which is substantially equal to said first working voltage value, said second working voltage value, or said third working voltage value is applied to the input terminal of one of the said resonant-tunnelling transistors of said logic gates, to latch one of the output terminals of said logic gates at one of said first, second or third output values, another of the output terminals of said logic gates at another of said first, second or third output values, and the other of the output terminals of said logic gates at the other of the said first, second or third output values, in dependence upon the value of said pulse signal. According to a third aspect of the present invention, there is provided a semiconductor memory device comprising a plurality of word lines, a plurality of bit lines, and a plurality of memory calls connected between respective word lines and bit lines, each memory cell comprising latching circuitry in accordance with the second aspect of the present invention and a first transfer gate, which first transfer gate is arranged and connected to transfer data, provided at an output terminal of one of the logic gates forming said latching circuitry, to a bit line associated with the memory cell concerned in response to a control signal delivered to the said first transfer gate via a word line associated with the memory cell concerned, the device further comprising a data bus line and a plurality of second transfer gates, which second transfer gates are associated with respective bit lines and are each arranged and connected to transfer data from the bit line associated therewith to the said data bus line in response to a further control signal. Reference will now be made, by way of example, to the accompanying drawings, in which: Figs. 1a and 1b show views representing a structure and an energy state of a resonant-tunnelling hot electron transistor (RHET); Fig. 1c shows a view representing an energy state of a resonant-tunnelling bipolar transistor (RBT); Figs. 2a to 2c are graphs illustrating an energy state of the resonant-tunnelling transistor in Fig. 1a; Fig. 3 is a graph of the characteristics of the resonant-tunnelling transistor in Fig. 1a; Fig. 4 is a circuit diagram of a logic gate embodying the first aspect of the present invention; Fig. 5 is a graph for use in explaining the operation of the logic gate in Fig. 4; Figs. 6a and 6b show views of latching circuitry embodying the second aspect of the present invention, Fig. 6b being a simplified circuit diagram thereof, and Fig. 6c is a truth table of that latching circuitry; Fig. 7 is a circuit diagram of a semiconductor memory device embodying the third aspect of the present invention; Fig. 8 is a graph for use in explaining the operation of the semiconductor memory device of Fig. 7; and Figs. 9 and 10 show views of modifications of the device of Fig. 7. Before describing the preferred embodiments of the present invention, a description will be given of the principle of a resonant-tunneling transistor device. Figure 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 Al y Ga 1-y As (e.g. y = 0.3) collector side potential barrier layer 2 on the collector layer 1, an n⁺ - type GaAs base layer 3, on the potential barrier layer 2, a superlattice structure 4, an n⁺ - type GaAs emitter layer 5, an emitter electrode 6, and a base electrode 7. The superlattice structure 4 consists of an Al x Ga 1-x As barrier layer 4A₁, a non-doped impurity GaAs quantum well layer 4B, and an Al x Ga 1-x As barrier layer 4A₂. The superlattice structure 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 E C represents a bottom of a conduction-energy band, and E X an energy level of a sub-band (resonant-state energy level) at the quantum well. Referring to Figs. 2a to 2c, the principle of operation of the resonant-tunneling transistor device will be described. Figure 2a is a graph of an energy band of the RHET device when a voltage V BE between the base layer 3 and the emitter layer 5 is lower than 2-E X /q, wherein q represents charge of 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 5, 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 almost zero, and thus a level of an energy E FE , namely 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  C represents a conduction-band discontinuity. Figure 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, elections 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 into kinetic energy, bringing the electrons to a so-called "hot" state. The hot electrons are ballistically passed through the base layer 3 and reach 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 the 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. In the known transistor, the resonant-tunnelling effect does not occur in these circumstances, and the electrons introduced from the emitter layer 5 to the base layer 3 do not exist, with a consequent drop in current flowing in the RHET device. In contrast, a logic gate embodying the first aspect of the present invention employs a resonant-tunnelling transistor in which, by decreasing the barrier height of the barrier layer 4A₁ adjacent to the base layer 3 sufficiently, the electrons need only tunnel through the barrier layer 4A₂ adjacent to the emitter layer 5. As a result, the collector current for V BE > 2E x /q is enhanced. Fig. 1c is a graph representing an energy band of a resonant-tunnelling bipolar transistor (RBT). A 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 E x . There is a PN junction at the interface between the base layer and the collector layer. Since the operation of a RBT also relies on resonant-tunnelling and is similar to that of the RHET, an explanation thereof is omitted. Fig. 3 is a graph representing 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 C₁ 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, each of which curves indicates n-shaped differential negative-resistance characteristics. Referring to Figs. 4 and 5, a ternary logic gate 1 embodying the first aspect of the present invention and the operation thereof will be described. The logic gate 1 includes 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 the collector of the resonant-tunnelling transistor 11. An emitter of the RHET 11 is grounded (V SS ). Another end of the resistor 13 is connected to a DC power supply having a voltage V CC . The signal source 12 outputs the voltage V IN having a working voltage value either of V INH which is higher than 2·E x /q or V INM which is approximately equal to 2·E x /q, or V INL which is lower than 2·E x /q. When the voltage V IN having a low value V INL is supplied to the base of the RHET 11, an output voltage V OUT at the collector of the RHET 11 is a high level V H ; when the median voltage V INM is supplied to the base of the RHET, the output voltage V OUT is low level V L ; and, when the high voltage V INH is supplied to the base of the RHET, the output voltage V OUT is median level V M . The truth table of the ternary logic circuit in Fig. 4 as is follows; V IN V OUT Low High Median Low High Median Referring back to Fig. 3, preferably the local minimum value of the collector current I C , i.e. when the input voltage is V INH , is arranged to be a half of the local maximum value of the collector current, i.e. when the input voltage is V INM . Fig. 6a shows a latch circuit which embodies the second aspect of the present invention and includes three logic gates 1A, 1B, 1C, as described above, i.e. three series and circular-connected RHETS 11A, 11B and 11C, and three resistors 13A, 13B, and 13C. The signal source 12 is connected to the RHET 11A. Fig. 6b is a simplified version of the circuit in Fig. 6a. When the voltage V IN 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 , high level voltage V OUT2 and 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 volt, the above voltage status is not changed. Thus, if a pulse voltage having an amplitude of V INL , V INM or V INH is supplied to the resonant-tunneling transistor 11A, 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. Fig. 7 is a partial circuit diagram of a semiconductor memory device which embodies the third aspect of the present invention and 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, BL and data buses DB, 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 RHET's 251 to 253, a write amplifier 27 having three RHET's 271 to 273, and a pair of gate transistors 26 between the data buses DB and DB and 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 time, and the ordinate represents signal voltage. References H, M, and L represent high level, middle level, and low level voltages of the resonant-tunneling transistor. First, the read operation will be described. The read operation is carried out during times 0 to t₁. Figure 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 BL was not fully completed, and therefore, a small voltage difference between the bit lines BL and BL still remains. Upon receipt of an address signal, from an address buffer (not shown) that has received the address signal, the row decoder 21 and 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 t₁, 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. Voltages from the latch circuit 241 are transferred on the bit lines BL and BL . The voltages on the bit lines BL and BL are 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 t₂, upon receipt of a sense strobe signal s, the sense amplifier 25 having a tristable 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 three 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 WE at the clock generator 30 at a time t₄ , the column gate transistors 23a and 23b are turned ON at a time t₅. 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, modified sense amplifier, emboding the third aspect of the present invention, is shown, which employs a single bit line BL and a single data bus DB. This circuit increases the integration of IC devices but has a lower operation speed. Referring to Fig. 10, another modified sense amplifier, embodying the third aspect of the present invention, is shown, which employs three bits lines BL and three data buses DB. 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 the conventional binary circuit. 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 at the conversion circuit. The capacity of the memory is only 1.5 times that of the binary, 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 preferably, because, for example, 64K cells substantially function as 96K cells.

Claims: A logic gate (1) comprising: a resonant-tunnelling transistor (11) having a base (3), an emitter (5) and a collector (1), there being between the base (3) and the emitter (5) a superlattice structure (4) providing a quantum well layer (4B) between first and second barrier layers (4A₁, 4A₂), said quantum well layer (4B) having a resonant-state energy level (E x ); and a resistor (13) connected in series, between first and second power supply lines (V cc , V ss ), with the emitter-collector current path of said resonant-tunnelling transistor (11); the base (3) of said resonant-tunnelling transistor (11) serving as an input terminal of the logic gate (1) for the application thereto of an input voltage signal (V IN ) constituting a base-to-emitter voltage of the said transistor when the gate is in use; and a circuit node between the said resistor (13) and the transistor (11) serving as an output terminal of the logic gate (1) for delivering an output voltage signal (V OUT ) when the gate is in use; whereby when the input voltage signal (V IN ) is increased, from a first working voltage value to a second working voltage value, current passing along the said emitter-collector current path is thereby caused to increase from a first current value to a second current value, and when the input voltage signal (V IN ) is increased from the second working voltage value to a third working voltage value such current is thereby caused to decrease from said second current value to a third current value; the said superlattice structure (4) being so formed that when the said input voltage signal (V IN ) has the said third working voltage value the potential height of one of said first and second barrier layers (4A₁, 4A₂) differs from that of the other sufficiently to ensure that said third current value lies between and is distinct from the said first and second current values, so that the said output voltage signal (V OUT ) has discrete first, second and third output values respectively when the said input voltage signal (V IN ) has the said first, second and third working voltage values, the said third output value of the output voltage signal (V OUT ) being between the said first and second output values thereof. A logic gate as claimed in claim 1, wherein the said third current value is approximately half the said second current value. A logic gate as claimed in claim 1 or 2, wherein said second working voltage value is approximately equal to 2E x /q, where E x is the resonant-state energy level, and q represents the charge on carriers in the said resonant-tunnelling transistor (11). Latching circuitry comprising three logic gates (1A, 1B, 1C), each gate being a logic gate as claimed in any preceding claim, the output terminal of each gate (1A, 1B, 1C) being connected to the input terminal of another of the gates such that the gates form a closed loop; said latching circuitry thereby being operable, when a pulse signal which is substantially equal to said first working voltage value, said second working voltage value, or said third working voltage value is applied to the input terminal of one of the said resonant-tunnelling transistors (11A, 11B, 11C) of said logic gates (1A, 1B, 1C), to latch one of the output terminals of said logic gates (1A, 1B, 1C) at one of said first, second or third output values, another of the output terminals of said logic gates (1A, 1B, 1C) at another of said first, second or third output values, and the other of the output terminals of said logic gates (1A, 1B, 1C) at the other of the said first, second or third output values, in dependence upon the value of said pulse signal. A semiconductor memory device comprising a plurality of word lines (WL), a plurality of bit lines (BL, BL ), and a plurality of memory cells (MC) connected between respective word lines (WL) and bit lines (BL, BL ) each memory cell (MC) comprising latching circuitry (241) as claimed in claim 4 and a first transfer gate (242, 243), which first transfer gate (242, 243) is arranged and connected to transfer data, provided at an output terminal of one of the logic gates forming said latching circuitry (241), to a bit line (BL, BL ) associated with the memory cell (MC) concerned in response to a control signal delivered to the said first transfer gate (242, 243) via a word line (WL) associated with the memory cell (MC) concerned, the device further comprising a data bus line (DB, DB ) and a plurality of second transfer gates (23a, 23b), which second transfer gates (23a, 23b) are associated with respective bit lines (BL, BL ) and are each arranged and connected to transfer data from the bit line (BL, BL ) associated therewith to the said data bus line (DB, DB ) in response to a further control signal. A device as claimed in claim 5, wherein each memory cell (MC) has an additional first transfer gate (242, 243) which is arranged and connected to transfer data, provided at an output terminal of another of the logic gates forming said latching circuitry (241), to a further bit line (BL, BL ) of said plurality of bit lines which is associated with the memory cell (MC) concerned, in response to said control signal which is delivered to the additional first transfer gate via the word line (WL) associated with the memory cell (MC) concerned, and wherein the device further comprises an additional data bus line (DB, DB ) and a further plurality of second transfer gates (23a, 23b), which second transfer gates of said further plurality are associated with respective further bit lines (BL, BL ) and are each arranged and connected to transfer data from the further bit line (BL, BL ) associated therewith to the said additional data bus line (DB, DB ) in response to said further control signal. A device as claimed in claim 6, further comprising a sense amplifier (SA) including latching circuitry as claimed in claim 4, the said data bus line (DB, DB ) and the said additional data bus line (DB, DB ) being connected respectively to transfer data to two of the three output terminals of the logic gates forming the latching circuitry (241) of said sense amplifier (SA). A device as claimed in claim 6 or 7, further comprising a write amplifier (WA) including latching circuitry (241) as claimed in claim 4, the said data bus line (DB, DB ) and the said additional data bus line (DB, DB ) being connected respectively to transfer data to two of the three output terminals of the logic gates forming the latching circuitry (241) of said write amplifier (WA).

Inventors: TAGUCHI, MASAO, YOKOYAMA, NAOKI
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: G11C11
IPC codes: G11C11, H01L21, F16L55, H01L29, H03K19, H03K3