Termination circuit for word lines of a semiconductor memory device
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a termination circuit for the word lines of a semiconductor memory device, more particularly to a termination circuit which can reduce both the rise time and the fall time for a word selection signal applied to the word lines. 
     (2) Description of the Prior Art 
     In general, it is necessary to decrease both the power consumption of each memory cell and the size of each memory cell in order to increase the capacity of a semiconductor memory device. One of the methods for decreasing the power consumption of each of the memory cells is to increase the resistance value of the load resistors of each of the memory cells so that the holding current passing through non-selected word lines is decreased. However, increasing the resistance value of the load resistors leads to an increase in the transition time from a selected condition to a non-selected condition, because the fall time of the word selection signal is increased. Consequently, the desired high speed operation of the memory device can not be attained. 
     In order to decrease the fall time of the word selection signal applied to a word line, a conventional memory device, as disclosed in the thesis &#34;AN ECL 100 K Compatible 1024×4b RAM with 15 ns Access Time&#34;, by Ulf Buerker, et al. , ISSCC DIGEST OF TECHNICAL PAPERS, Feb. 15, 1979, p. 102, includes a termination circuit as illustrated in FIG. 1. 
     In FIG. 1, a memory cell array MCA composed of a plurality of memory cells MC ij  (i=1,2, . . . , n; j=1,2, . . . , m) whose matrix is n rows by m columns is illustrated. Each of the memory cells MC ij  is a well known flip-flop type cell. The memory cell, for example MC 11 , includes two multiemitter transistors T 1  and T 2 , two load resisters R 1  and R 2 , and two diodes D 1  and D 2 , which are connected parallel to the load resisters R 1  and R 2 , respectively. One of the emitter electrodes of the multiemitter transistor T 1  is connected to a hold line HL 1  and the other emitter electode of the multiemitter transistor T 1  is connected to a bit line B 1 . One of the emitter electrodes of the multiemitter transistor T 2  is connected to the hold line HL 1  and the other emitter electrode of the multiemitter transistor T 2  is connected to a bit line B 1 . The word lines WL 1 , WL 2 , . . . , WL n  are connected to word line drivers WD 1 , WD 2 , . . . , WD n  respectively. The hold lines HL 1 , HL 2 . . . , HL n  are connected to hold current sources 1 1 , 1 2 , . . . , 1 n  respectively. The hole lines HL 1 , HL 2 , . . . , HL n  are also connected to the anode electrode of diodes D 11 , D 12 , . . . , D 1n , respectively, of a termination circuit 1, and the cathode electrodes of the diodes D 11 , D 12 , . . . , D 1n  are commonly connected to an additional current source 10. 
     One of the memory cells MC ij , for example the memory cell MC 11 , is selected by applying a high potential word selection signal to the word driver WD 1  and a high potential bit selection signal to a bit line driver connected to the bit line pair B 1  and B 1 , not shown in FIG. 1. Therefore, the potential of the selected word line is high and the potentials of the non-selected word lines are low. The potential of the hold line is lower than the corresponding word line by a nearly constant value, i.e., the base-emitter voltage of one of the transistors in the memory cell. 
     The termination circuit 1 of the memory device of FIG. 1 helps to reduce the fall time of the potential of the word line, for example WL 1 , whenever the word line changes its state from the selected condition to the non-selected condition, by adding the current ΔI H  to the current I H  flowing from the selected word line WL 1  through the memory cells MC 11 , . . . , MC 1m  to the hold line HL 1 . Since the potential level of the hold line HL 1 , corresponding to the selected word line WL 1 , is higher than that of the other hold lines HL 2 , . . . , HL n , the diode D 11  connected to the selected hold line HL 1  is turned on and the current ΔI H  flows from the selected hold line HL 1  through the diode D 11  to the additional current source 10. Consequently, the potential level of the hold line HL 1  and, thus the potential level of the word line WL 1  falls quickly. The additional current ΔI H  serves to quickly discharge the electric charge stored in the stray capacitance included in the word line, the hold line and the memory cells connected between the word line and the hold line, so that the potential drop of the word line is accelerated. 
     In order to decrease the operation time of a memory device, for example, the read out time, it is necessary to quickly lower the output potential of a non-selected memory cell. This quick reduction of the output potential of a non-selected memory cell is necessary because a read out current flows from the non-selected memory cell to the bit line even when the potential level of the non-selected memory cell is high, and a read out from the next selected memory cell cannot begin before the potential level of the non-selected memory cell drops. The potential level of the non-selected memory cell does not fall quickly, even if the potential level of the word line falls quickly, because of stray capacitance contained in the memory cell. Therefore, as mentioned above, the potential of the memory cell is forced to drop to a low level by the additional current ΔI H  flowing from the memory cell through the hold line to the additional current source 10. It should be noted that the additional current flow is required temporarily during the transition period from the selected condition to the non-selected condition. 
     FIG. 2 illustrates graphically the relationship between the potential of two word lines, one which is, for example, the word line WL 1  which had been previously selected and which is changing its potential from high (selected condition) to low (non-selected condition); and the other word line which is, for example, the word line WL 2  which was selected after the word line WL 1  and which is changing its potential from low to high. The dotted lines in FIG. 2 illustrate the characteristics of the potential of the two word lines WL 1  and WL 2  when the termination circuit 1 of FIG. 1 is not used, and the solid lines illustrate the potential of the two word lines WL 1  and WL 2  when the termination circuit of FIG. 1 is used. As shown in FIG. 2, a cross point CP of the solid lines comes faster than a cross point CP&#39; of the dotted lines because of the termination circuit 1. 
     Although the termination circuit 1 can reduce the fall time of the potential of the word line, for example, WL 1  until the cross point CP is reached, the fall time after the cross point CP cannot be reduced. The further reduction cannot be accomplished because the additional current ΔI H  does not flow from the hold line HL 1  but flows from the hold line HL 2  to the additional current source 10 after the cross point CP has been reached. That is, among the diodes D 11 , D 12 , . . . , D 1n  of the termination circuit 1, only the diode which is connected to the hold line having the highest potential turns on. Until the cross point CP is reached the diode D 11  is turned on and after the cross point CP is reached the diode D 12  is turned on. Therefore, until the cross point CP is reached, the additional current ΔI H  flows from the hold line HL 1 , corresponding to the word line WL 1  which had been previously selected; and after the cross point CP, the additional current ΔI H  flows from the hold line HL 2 , corresponding to the word line WL 2  which was selected subsequent to the word line WL 1 , so that only the hold current I H  flows through the hold line HL 1 . 
     FIG. 3 illustrates other conventional termination circuits 2 1 , 2 2 , . . . , 2 n , as disclosed in the thesis &#34;A 6 ns 4 Kb Bipolar RAM using Switched Load Resistor Memory Cell&#34;, by Masaaki Inadachi, et al., ISSCC DIGEST OF TECHNICAL PAPERS, Feb. 15, 1979, p. 108. The termination circuit, for example, 2 1  reduces the fall time for the potential level of the word line WL 1  by delaying the trailing edge of the word selection signal on the word line WL 1  and not delaying the leading edge of the word selection signal on the word line WL 2 , so that the additional current ΔI H  flows from the hold line HL 1 , corresponding to the word line WL 1 , to the current source ΔI H  until the potential level of the word line WL 1  falls to a substantially low level, even after the word line WL 1  has changed its state from the selected condition to the non-selected condition. 
     One of the termination circuits 2 1 , 2 2 , . . . , 2 n  of FIG. 3, for example, the termination circuit 2 1 , includes an NPN type transistor T 31 . The collector electrode of transistor T 31  is connected to the word line WL 1  through a resistor R 31  and a capacitor C 31  connected in parallel and, the emitter electrode is connected to a negative voltage source line NL through a resistor R 33  and the base electrode is connected to a reference voltage source V REF  having a constant voltage, causing the transistor T 31  to operate as a constant current source. The termination circuit 2 1  also includes an NPN type transistor T 32 , the base electrode of transistor T 32  is connected to the collector electrode (point A) of the transistor T 31 , the collector electrode is connected to the ground and the emitter electrode is connected to the negative voltage source line NL through a resistor R 32  and a capacitor C 32  connected in parallel. The termination circuit 2 1  further includes an NPN type transistor T 33 , the base electrode of transistor T 33  is connected to the emitter electrode (point B) of the transistor T 32 , the collector electrode of transistor T 33  is connected to the hold line HL 1  and the emitter electrode is connected to the negative voltage source line NL through a resistor R 34 . 
     In the termination circuit 2 1  of FIG. 3, the potential of point A rises substantially at the same time as the potential rise of the word line WL 1 , due to the action of the capacitor C 31 , and the potential of the point B, which is lower than that of point A by the base-emitter voltage V BE  of the transistor T 32 , also rises at substantially the same time as the potential rise of the word line WL 1 . Therefore, transistor T 33  turns on immediately after the potential rise of the selected word line WL 1 , and the additional current ΔI H  flows from the hold line HL 1  through the transistor T 33  and the resistor R 34  to the negative voltage source line NL. When the potential of the word line WL 1  drops, due to the transition from the selected condition to the non-selected condition, the potential of the point B does not fall immediately but falls slowly, due to the action of the capacitor C 32 , so that the transistor T 33  does not turn off immediately after the drop of the potential of the word line WL 1  but continues in a turned on condition for a short time. Therefore, as shown in FIG. 4, the additional current ΔI H  continues to flow through the hold line HL 1  for a predetermined time period Δt, even after the potential V W  of the word line WL 1  has fallen from the high level (selected condition) to the low level (non-selected condition). Consequently, the fall time for the potential V W  of the word line is reduced as shown by a solid line in FIG. 4. In FIG. 4, dotted lines show the characteristics of the potential V W  of the word line and the additional current ΔI H  of the circuit of FIG. 1. 
     However, in the termination circuit 2 1  of FIG. 3, the transistor T 33  turns on immediately after the potential rise of the word line WL 1  and the additional current ΔI H  flows immediately through the hold line HL 1 , corresponding to the word line WL 1 . Therefore, the load of the word line WL 1 , i.e., the load on the word line driver WD 1 , becomes heavy during the potential rise of the word line WL 1 , causing the rise time for the potential V W  of the word line WL 1  to be increased when the word line driver WD 1  does not have a large driving capability. 
     The termination circuits 2 1 , 2 2 , . . . , 2 n  of FIG. 3 also have the disadvantage that each of the termination circuits 2 1 , 2 2 , . . . , 2 n  of FIG. 3 uses many circuit parts, including two capacitors, for example, C 31  and C 32 , so that the area occupied by each of the termination circuits 2 1 , 2 2 , . . . , 2 n  in an integrated circuit of a memory device becomes large. Another disadvantage is that a termination circuit 2 1 , 2 2 , . . . , 2 n-1  or 2 n  is provided for each of the word lines of a memory device, and therefore, a memory device of, for example, 4 Kbits (64×64 bits), which has 64 word lines, needs 64 termination circuits, thus requiring a large area for the termination circuits. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce the rise time and the fall time for the potentials of word lines of a memory device as compared to the prior art. 
     It is another object of the present invention to provide a termination circuit consisting of a small number of circuit parts. 
     It is another object of the present invention to provide a termination circuit which does not occupy a large area in the integrated circuit of a memory device. 
     According to the present invention, there is provided a termination circuit for a static semiconductor memory device including, for each word line, a first transistor which detects potential change of a word line; a delay circuit which delays an output signal from the first transistor by a predetermined time period and a second transistor which is turned on and off by an output signal from the delay circuit, each of the second transistors being connected between one of the hold lines and a common hold current source, and the second transistors and the common hold current source composing a current switch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block circuit diagram illustrating a memory device having a conventional termination circuit. 
     FIG. 2 is a timing diagram graphically illustrating the transition of the potentials of the word lines in the memory device of FIG. 1. 
     FIG. 3 is a block circuit diagram illustrating a memory device having other conventional termination circuits. 
     FIG. 4 is a timing diagram graphically illustrating the characteristics of a potential V W  of a word line and the characteristics of an additional current ΔI H  in the memory device of FIG. 3. 
     FIG. 5 is a block circuit diagram illustrating a memory device having a termination circuit according to the present invention. 
     FIG. 6 is a timing diagram illustrating signals appearing in the memory device of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 5 illustrates an embodiment of a memory device including a termination circuit in accordance with the present invention. The same parts appearing in FIG. 5 as appear in FIG. 1 or FIG. 3 are designated by the same reference symbols and an explanation thereof is omitted herein. 
     In FIG. 5, X 1 , X 2 , . . . , X n  designate word address signals which are applied to the word line drivers WD 1 , WD 2 , . . . , WD n , respectively, and each of which becomes high in the selected condition. Y 1 , Y 2 , . . . , Y m  designate bit address signals which are applied to bit line drivers BD 1 , BD 2 , . . . , BD m , respectively, and each of which becomes high in the selected condition. One of the bit line drivers BD 1 , BD 2 , . . . , BD m  to which a high level bit address signal is applied turns on. The termination circuit 3, according to the present invention, comprises n termination circuit units 3 1  through 3 n , each of which is connected to end points of the corresponding one of the word lines WL 1  through WL n  and of the corresponding one of the hold lines HL 1  through HL n . Each of the termination circuit units 3.sub. 1 through 3 n  has the same structure, and therefore, only the termination circuit unit 3 1  connected to the word line WL 1  and the hold line HL 1  will now be explained. The termination circuit unit 3 1  comprises: a first NPN type transistor T 11 , which operates as an emitter follower having a base electrode connected to the word line WL 1  ; a delay circuit 4 1 , which delays an output signal from the transistor T 11  and which consists of a resistor R 11  and a capacitor C 11  ; a second NPN type transistor T 21 , which receives an output signal from the delay circuit 4 1 , having a collector electrode connected to the hold line HL 1  and having an emitter electrode connected commonly to a common hold current source 30; and a resistor R 21  connected between the base electrode of the transistor T 21  and the common bias current source I BS . The resistors R 21  and R 11  and the common bias current source I BS  determine a base bias potential for the transistor T 21 . It should be noted that n NPN type transistors T 21  through T 2n  and the common hold current source 30 comprise a current switch. 
     Operation of the termination circuit 3 in FIG. 5 will now be described. When the word line, for example, WL 1  changes from the selected condition to the non-selected condition or from the non-selected condition to the selected condition, the potential of the word line WL 1  changes from high level, for example -0.9 V, to low level, for example -1.7 V, or from low level to high level, respectively. Such a potential change is transferred, after a short time delay, to the base electrode of the transistor T 21  by the transistor T 11  and the delay circuit 4 1  with the length of the time delay determined by a time constant R 11  ·C 11 . 
     Assuming that the word line, for example WL 1 , has been previously selected and is changing from the selected condition to the non-selected condition, and another word line, for example WL 2 , has been selected after the word line WL 1  and is changing from the non-selected condition to the selected condition, the potential V W  of the word line WL 1  changes from high to low and the potential V W  of the word line WL 2   changes from low to high as illustrated graphically in FIG. 6. A potential V F  of point F 1 , that is, the output of the delay circuit 4 1  changes from a relatively high level to a relatively low level after a short time delay relative to the potential change in word line WD 1 , and the potential V F  of point F 2 , that is, the output of the delay circuit 4 2  changes from a relatively low level to a relatively high level after a short time delay relative to the potential change in word line WD 2  as shown in FIG. 6. Therefore, cross point CP 2  of the potentials of the points F 1  and F 2  is later than cross point CP 1  of the potentials of the word lines WL 1  and WL 2  by a delay time Δt, which is determined by the time constant R 11  ·C 11 . Practical experiments have shown that the delay time Δt amounts to about 4 ns in the condition of C 11  =1 P  F, R 11  =R 21  =5 KΩ and the delay time can be set to any value by selecting values of the capacitor C 11  and the resistors R 11  and R 21 . Since the transistors T 21  through T 2n  compose the current switch, only the one of the transistors T 21  through T 2n  which has the highest base potential is turned on at any one time. Therefore, until the cross point CP 2  is reached, the transistor T 21  is in the turned on condition and the additional current ΔI H  flows from the hold line HL 1  through the transistor T 21  to the common hold current source 30, and after the cross point CP 2  the transistor T 22  is turned on and the additional current ΔI H  flows from the hold line HL 2  through the transistor T 22  to the common hold current source 30. 
     Since the additional current ΔI H  continues to flow through the hold line HL 1  until a short time after the word line WL 1  has changed from the selected condition to the non-selected condition, the fall time of the potential of the word line WL 1  is reduced. Since the additional current ΔI H  does not flow through the hold line HL 2  immediately after the word line WL 2  has changed from the non-selected condition to the selected condition, excessive current does not flow from the word line driver WD 2  through the word line WL 2  to the hold line HL 2  at the transition period of the potential of the word line WL 2  from low to high, so that the rise time for the potential of the word line WL 2  is reduced. 
     In the above described embodiment, the common bias current source I BS , which is common to a plurality of termination circuit units 3 1  through 3 n , is used. However, it is also possible to use a plurality of bias current sources, each of which is connected to the base electrode of one of the transistors T 21  through T 2n  respectively. 
     According to the present invention, the switching point of the additional current ΔI H  is delayed from the switching point of the word line potential V W  by using the delay circuit comprising, for example, the resistor R 11  and the capacitor C 11 , so that both the rise time and the fall time of the word line potential can be reduced. Since the bias current I BS  can be decreased to a relatively small value without affecting the operation of the termination circuit, the resistance value of the resistors R 11  and R 21  can be increased so that a large delay time can be obtained. Therefore, the capacitance of the capacitor C 11  can be decreased so that the area occupied by the capacitor C 11  can be very small. In addition, the termination circuit, according to the present invention, uses a small number of circuit parts, each of the termination circuit units using only one capacitor, for example C 11 , so that the entire termination circuit occupies a relatively small area in the integrated circuit of a memory device.