Semiconductor memory device.

0001This invention relates to a semiconductor memory device, particularly to a memory device having a circuit which provides the first transistor and gives a discharge current to a word line via said first transistor.

0002A memory device, providing so-called the emitter detection type memory cell where a pair of npn type bipolar transistors are cross-coupled and the emitters of such pair of transistors are connected to a pair of bit lines, is often used. A memory device of this type facilitates the coupling between peripheral circuits constituted by the ECL (Emitter Coupled Logic) circuits and memory cells and simultaneously realizes high speed readout operation. These are outstanding advantages of this memory device. Moreover, diversified improvements are attempted in order to realize further high speed readout operation. For example, the reference readout level given to a transistor which connects the sense amplifier and bit line is temporarily lowered when a potential of the word line changes. This method is mainly intended to curtail the rise time of the potential of the word line, but as a method for curtailing the fall time of the potential of the word line, the fall time of the potential of the word line is shortened by giving a discharge current to the selected word lines of memory which are to be operated under larger capacity and lower current.

0003In regard to the first word line (W⁺) and the second word line (W^-) to which a memory cell is connected, the first word line (W⁺) is provided with the detecting means which detects a potential of the first word line (W⁺) and the delay means which delays a detected output, while the second word line (W^-) is provided with the discharge means which allows a discharge current to flow from the first word line (W⁺) via the memory cell, and second word line (W ) in accordance with an output of said delay means.

0004In such a structure, when the selected word line is to be shifted to the non-selected condition, said discharge means turns ON in accordance with the detected output of said detecting means, allowing the discharge current to flow. As the word line turns into the non-selected condition, the potential of the first word line (W⁺) is lowered and when it becomes lower than the specified voltage, no detected output can be obtained.

0005However, said delay means causes a detected output to be applied to the discharge means even when the first word line (W⁺) is set to a potential which is lower than the specified value and as a result a discharge current flows continuously for some specified period.

0006Thereby, the first word line voltage is rapidly lowered and the fall time of word line potential is curtailed.

0007In such existing memory device, on the occasion that the word line is to be shifted to the selected condition from non-selected condition, when a voltage of the first word line (W⁺) gradually rises and exceeds the specified voltage, said detecting means makes said discharge means ON and the discharge current starts to flow during the word line voltage rises.

0008Since this discharge current flows into the transistor which is turned ON among the cross-coupled transistors within the memory cell from the first word line (W⁺), a large amount of charges are stored in the base and collector of transistor which is turned ON.

0009Therefore, when inverted data is written into this memory cell, in other words, when the transistors which are turned ON among the cross-coupled transistors are turned OFF, a large amount of charges being stored must be absorbed. For this purpose, it is required to give a heavy write current to the memory cell.

0010Moreover, a heavy discharge current on the first word line (W⁺) brings about reduction of word line potential due to a voltage drop of the word line. Further, a heavy discharge current flowing via the first word line (W⁺) causes a heavy base current to flow into the word line driver transistor, resulting in reduction of base potential due to a voltage drop by the resistor existing between the base of word line driver transistor and ground. Thereby, a potential of word line which is lower than the "base potential by V_BE of the word line driver transistor is further lowered.

0011When the discharge current flows as explained above, the voltage of the first word line (W⁺) is lowered and as a result the voltage of the selected word lines comes close. to the voltage under the non-selected condition, thus reducing a noise margin.

0012It is an object of the present invention to provide a semiconductor memory device which assures short fall time of the word line potential without allowing the write current of memory to increase. It is another object of the present invention to provide a semicon_- ductor memory device which assures sufficiently large noise margin.

0013In order to attain the above-mentioned objects, the present invention discloses a semiconductor memory device providing a circuit for applying the discharge current to the selected word lines, wherein a circuit for detecting a change of the address signal is provided and thereby said discharge current is temporarily applied only when said selected word lines are shifted to the non-selected condition.

0014Various embodiments of methods and apparatus in accordance with this invention will now be described and contrasted with the prior art with reference to the accompanying drawings; <ul id="ul0001" list-style="none"><li>Fig. 1 shows an embodiment of the present invention;</li><li>Fig. 2A shows potential change of node B of Fig. 1;</li><li>Fig. 2B shows potential change of node C of Fig. 1;</li><li>Fig. 2C shows potential change of word line of Fig. 1.</li></ul>

0015Figure 1 shows an embodiment of the present invention. DEC 1... DECn are decoders; WD1.....WDn are word line drivers; W1+ " ,Wn+ are positive word lines; W₁^- .....Wn^- are negative word lines (hold lines); B₁, <o>B</o>₁,...B_m, <o>B</o>_m are bit lines; MC₁₁....MC_nm are memory cells of emitter detection type to be connected to these lines. The decoders DEC₁.... DEC_n respectively have plurality of diodes d. The signals <o>A</o>₁, A₁,A₂, A₂, .... are generated by the input gates IG₁, IG₂, ...IG_n corresponding to each address input AI₁, AI₂,...., AI_n and selectively applied to the diode d. As is well known, the word lines W_i⁺ are selected through the one word line driver among WD_{1 "},.WD_nonly when all inputs for plural diodes within each decoder are H (high) level. This condition is established simultaneously for only one decoder, at least one input of remaining decoders is L (low) level and the word lines corresponding to these are all kept at the non-selected level.

0016^T₁ is the positive word line W_i⁺(i = 1, 2, ..., n) level detection transistor and its emitter output charges the time constant circuit RC becomes the base voltage of discharge transistor T₂ which is connected to the negative word line W_i^-. These transistors T₁, T₂ and time constant circuit RC are provided for each word line and the current switch (discharge circuit DIS) for discharge current I_DIS is formed with the emitters of all transistors T₂ connected in common.

0017In such discharge circuit DIS having said structure, since the point E of the time constant circuit RC connected to the selected word line W_i+ becomes highest in level, the corresponding transistor T2 turns ON, allowing the discharge current I_DIS to flow into the negative word line W_i^-. This invention allows such current I_DIS to flow temporarily to flow only when the selected word line W_i⁺ shifts to the non-selected level from the selected level. In other words, while said selected word line W_i⁺ rises to the selected level from the .non-selected level and the selected level is maintained continuously under the normal condition, the current _IDIS is not applied to the negative word line Wi.

0018In practical, a control transistor T₂ is additionally provided to the current switch for the current I_DIS and the base potential C is always set higher than the potential of point E of the time constant circuit RC connected to the selected word line. Thereby, the current I_DIS normally flows into the transistor T₃. The potential of point E of the selected word line W_i⁺ side becomes highest only during the potential of point C is lowered, the current I_DIS flows into the negative word line W_i^- through the transistor T₃,

0019The base potential C is generated through the route from the address change detecting circuits Z_{1 "}, Z_n to the delay/shaping circuit DLY. The address change detecting circuit is provided for each address input. Z₁ is provided for the address input AI₁ and is composed of the current switch consisting of transistors T₄ to T₆. Zn is provided for the input AI_n, Since the reference voltage V_R2 is set between the voltage at which the waveforms cross when the signals Ai₁ A_i change over and the H level of signal <o>A</o>_i, either transistors T₄ or T₅ turns ON under the normal condition where the address input does not change. Thus, the current switch gives the current I_B thereto.

0020When the address input AI_i changes, the conditions, VR2 A_i and V_R2> A_i are satisfied temporarily and therefore the current I_B flows into the resistor R₁ through the transistor T₆. Since the collectors of transistors T₆ in the other address change detecting circuits are all connected to the connecting point B of the collector of transistor T₆ and the resistor R₁, if any of the address inputs AI₁, AI2, changes, a current flows into the resistor R₁, When the current I_B flows into the resistor R₁, the potential of point B temporarily lowers as shown in Fig. 2A. The diode D provided in parallel with the resistor R₁ is the clamping diode which prevents fluctuation of potential at the point B which may occur due to a difference of current flowing into the resistor ^R₁.

0021As will be obvious from above explanation, reduction of potential at the point B forecasts a change of voltage of the word line which may occur thereafter. W_i⁺ of Fig. 2C indicates the potential change of the word line which shifts from the selected condition to the non-selected condition, while W_j+ indicates that word line which shifts from the non-selected to selected condition. As shown in the operating waveform of the same figure, drop of the potential at the point B generally does not cover the cross point of potential on the word lines W_i⁺ and W₄+. Therefore, after the potential of the point B is shifted by the transistor T₇₁ the width of the wave form of the potential is enlarged by the delay/shaping circuit DLY consisting of the adequate number of AND gates AND and the OR gates OR, then an output of such DLY is used as the base potential for controlling the transistor T₃ explained above. Thereby, only during the short period t where the base potential C becomes low level, namely only in the hatched period where the selected word line W₁⁺ is shifted to the non-selected level, the discharge current I_DIS flows into the corresponding negative word line. As a result, the discharge current ^IDIS accurately flows only when the potential of word line falls. This is the most important condition. Thereby, the voltage of the selected word line rises quickly and said discharge current I_DIS does not flow when the selected word line voltage rises and during the normal condition. Thus, the write current no longer increases or noise margin also does not decrease.