SHIFT REGISTER

Abstract: A shift register having a simple circuit structure and used, for example, in a dynamic RAM device for a refresh operation. The shift register comprises a plurality of circuit stages mutually connected in cascade. Each of the circuit stages comprises: a first transistor (Q₄) for a transfer gate which is turned on and off by a first clock signal (⊘₀) and to which is input the output signal (OUT) of the previous circuit stage; a second transistor (Q₅) whose gate electrode is con­ nected to the output of the first transistor, whose drain or source electrode receives a second clock signal (⊘₁) hav­ ing a different phase from the first clock signal, and whose source or drain electrode outputs an output signal; and a reset means (Q₆, C₂) for rendering the input portion of the first transistor to a reset condition on the basis of the output signal, thereby sequentially transmitting data through each circuit stage.

Description: The present invention relates to a shift register, and more particularly to a MIS dynamic shift register which can be composed by using a small number of circuit elements. In a MIS dynamic memory device, for example, a so called auto-refresh operation for refreshing data stored in each memory cell is effected by memorizing the number (address) of a word line to be refreshed in the memory device, and by incrementing the number of the word line and refreshing the memory cells connected to each word line. In Japanese Patent Application No. 58-115887 filed on June 29, 1983 by the present applicant, there is disclosed a memory device which comprises a shift register disposed parallel to a row decoder and in which a selecting signal for a word line to be refreshed is generated using an output signal of the shift register, thereby sequentially selecting a word line to be refreshed. The shift register used in such a memory device is characterized in that, among all the bits thereof, the output of only one bit is "1" and the outputs of other bits are all "0". By using the output of the one bit which is "1", only one target word line is selected. It is also necessary that the width of one circuit stage on a semiconductor substrate of such a shift register be not larger than the pitch length of the word lines. Generally, since the pitch length of the word lines is very narrow, it is necessary that the structure of each circuit stage of the shift register be very simple and the number of circuit components used therein be small. In a conventional shift register which can memorize and transmit data having any pattern, each circuit stage has a very complex circuit structure and a large number of circuit components. Therefore, the circuit width of each circuit stage on a semiconductor device is relatively large, and it is impossible to use such a shift register in the above-mentioned memory device. In view of the problems of the aforementioned conventional shift register, the present invention adopts an idea of constituting each circuit stage in a MIS dynamic-type shift register by using three MIS transistors as base components. GB-A-2 022 953 discloses a shift register having features corresponding to those of the preamble of accompanying claim 1. According to the present invention, there is provided a shift register having a plurality of circuit stages connected in cascade and receiving first and second clock signals different in phase from each other for driving the shift register, each of said circuit stages comprising: transfer means operatively connected to receive an input signal and said first clock signal including a first transistor having a source or drain for receiving said input signal; a second transistor having a gate operatively connected via said first transistor to receive said input signal, a source and a drain, one of said source and drain receiving said second clock signal, said second transistor providing an output signal at the other of said source and drain in response to said input signal and said second clock signal; and a reset means for resetting said output signal; characterised in that when said first clock signal is applied to said transfer means, said transfer means allows the transfer of said input signal and wherein said first clock signal or said transfer means is arranged such that when said first clock signal is not activated, the gate voltage of said first transistor is held between Vth and 2Vth; and in that said reset means operates in response to said output signal of a succeeding circuit stage of the shift register, the output signal of the circuit stage providing the input signal applied to the first transistor of the succeeding circuit stage of the shift register. Above, Vth denotes the threshold voltage of the first transistor. An embodiment of the present invention can simplify the circuit structure of each circuit stage and decrease the number of circuit components used in each circuit stage, thereby decreasing the width of the space occupied by each circuit stage on a semiconductor substrate. Reference is made, by way of example, to the accompanying drawings in which: Figure 1 is a block circuit diagram illustrating a memory device which uses a shift register according to the present invention; Figure 2 is an electric circuit diagram illustrating a shift register as an embodiment of the present invention; Figure 3 is a waveform diagram illustrating the operation of the shift register of Fig. 2; and Figures 4 and 5 are electric circuit diagrams each illustrating another embodiment of the present invention and each being a modification of the shift register of Fig. 1. Figure 1 illustrates a dynamic memory device wherein a shift register, possibly a ring counter type shift register, is used to sequentially designate a word line connected to memory cells for the purpose of a refresh operation. The memory device of Fig. 1 comprises a memory cell array MCA having a plurality of memory cells MC each being connected to a bit line BL and a word line WL, a column decoder CD, a word decoder WD, and the above-mentioned shift register SR. The word decoder comprises a NOR gate NG for decoding address signals and an electronic switch SW for every word line WL. In the memory device of Fig. 1, an access operation such as a readout operation or write-in operation is effected in a state in which the switch SW is positioned so as to connect the output of the NOR gate NG to the corresponding word line WL. The word line WL is selected by a decoded signal, i.e., the output signal of the NOR gate NG, and the bit line BL is selected by the column decoder CD. Thus, the memory cell MC is selected and the access thereto is performed in a known manner. The switch SW is controlled by a refresh enable signal RFE of, for example, a high potential level so as to connect each of the output terminals of the shift register SR to the corresponding word lines. The shift register SR shifts a data "1" cyclically, and selects a word line sequentially, thereby refreshing memory cells connected to each word line. Figure 2 illustrates a structure of a shift register as an embodiment of the present invention used, for example, in the memory device of Fig. 1. In Fig. 2, the circuit portion surrounded by a dot-dash line constitutes a circuit of one stage of the shift register, and comprises three MIS transistors Q₄ , Q₅ , and Q₆ , and one capacitor C₂. The shift register of Fig. 2 is controlled by two kinds of clock pulses ø₀ and ø₁ , and these clock pulses ø₀ and ø₁ have phases mutually different, for example, by 180°, as shown in Fig. 3. The clock pulse ø₁ has an amplitude extending between power supply voltages V SS and V CC . However, the voltage of a low level portion of the clock pulse ø₀ is within the range of Vth to 2Vth and is preferably 1.5 Vth, although the voltage of a high level portion thereof is V CC ; where Vth is a threshold voltage of the MIS transistors, for example that of Q₄. Assume that the potential of an output node N2 of the pre-stage circuit is high, as shown in Fig. 3. At a time t₀ , the transistor Q₄ is turned on in response to the rising of the clock pulse ø₀ from a low level, e.g. 1.5 Vth, to a high level. Hence the potential of a node N3 rises to a high level, and the transistor Q₅ is turned on. At a time t₁ , the clock pulse ø₀ falls and the clock pulse ø₁ rises from a low level, i.e., V SS to a high level, i.e. V CC , so that node N4 rises to a high level because the transistor Q₅ is in the on-state. At this time, (t₁) the potential of the node N3 rises to a higher potential level in response to the potential rise of the clock pulse ø₁ due to the bootstrap effect of the transistor Q₅ , so that the node N4 rises to V CC level as well as the clock signal ø₁. When the potential of the node N4 rises, a transistor of the preceding stage Q₃ is turned on, and the potential of an output node N2 of the pre-stage circuit falls to a low level. At this time, the potential of the clock pulse ø₀ is at a low level, but since the low level potential of the clock pulse ø₀ is 1.5 Vth as mentioned before, when the potential of the node N2 becomes lower than 1/2 Vth, the transistor Q₄ is turned on, thereby pulling down the potential of the node N3 rapidly. In this way, transmission of the high level potential from the node N2 to node N4 is effected, and, similarly, the high level potential is sequentially transmitted through each circuit stage. The capacitor C₂ is provided so as to decrease the voltage loss caused when electric charges are transmitted from one stage to a succeeding stage, by rendering the capacitance of the output node N4 sufficiently larger than that of the input circuit, i.e. the node N5 of the succeeding stage. Since the low level voltage of the clock pulse ø₀ is 1.5 Vth, when, for example, the potential of the node N4 is at a high level and the clock pulse ø₀ is at a low level, i.e., 1.5 Vth, the potential of the node N5 of the next stage becomes 0.5 Vth and it is possible to keep a transistor Q₈ of the succeeding stage in a cut-off state. Therefore, it is possible to maintain the potential of a node N6 at a low level, i.e., V SS . If the low level potential of the clock pulse ø₀ is equal to or larger than 2 Vth, the potential of the node N6 disadvantageously rises to a value equal to or larger than V SS in response to the rise or fall of the clock pulse ø₁. It is necessary that the low level voltage of the clock pulse ø₀ is higher than Vth and lower than 2 Vth. Considering the variations in characteristics of each MIS transistor, it is preferable that the low level voltage of the clock pulse ø₀ is a value near 1.5 Vth. Figure 4 illustrates a circuit of one stage of a shift register as another embodiment of the present invention. The circuit of Fig. 4 differs from that of Fig. 2 in that a transistor Q₄' is connected in parallel to the input transfer gate transistor Q₄ , a voltage V B having a voltage of approximately 1.5 Vth is applied to the gate of the transistor Q₄', and the clock pulse ø₀ in Fig. 2 is replaced by a clock pulse ø₀'. The low level voltage of the clock pulse ø₀' is V SS which is the same as that of the clock pulse ø₁ , and the clock pulse ø₀' has the same waveform as that of a pulse formed by shifting the phase of the clock pulse ø₁ by 180°, for example. The other portions of Fig. 4 are the same as those of the circuit of Fig. 2 and are designated by the same reference symbols. In the circuit of Fig. 4, the parallel circuit of the transistors Q₄ and Q₄' effects the same operation as that of the transistor Q₄ of Fig. 2. That is, when the clock pulse ø₀' is low, the transistor Q₄ is turned off and the nodes N2 and N3 are connected only via the transistor Q₄'. Since the voltage V B of 1.5 Vth is applied to the gate of the transistor Q₄', the transistor Q₄' becomes equivalent to the transistor Q₄ when the clock pulse ø₀ in the circuit of Fig. 2 is in a low potential level, i.e., 1.5 Vth. In the circuit of Fig. 4, it is not necessary to strictly regulate the voltage level of the clock pulse ø₀', and it is advantageously possible to enlarge the allowable error of the voltage level of the clock pulse ø₀'. Figure 5 illustrates a circuit of one stage of a shift register as still other embodiment of the present invention. The circuit of Fig. 5 further comprises transistors Q₆' and Q₆'' connected to the transistor Q₆ in the circuit of Fig. 2. The transistors Q₆ and Q₆'' are mutually cross-coupled at the gates and the drains, and a signal from the output node N6 of the succeeding stage is input to the drain of the transistor Q₆'', i.e., a node N4' via the transistor Q₆'. The other portions are the same as those of the circuit of Fig. 2 and are designated by the same reference symbols. In the circuit of Fig. 5, a high level voltage of, for example, the node N4 is transmitted to the succeeding stage circuit, and when the output node N6 of the succeeding circuit becomes high, the potential of the node N4', i.e., the gate of the transistor Q₆ becomes high, thereby clamping the potential of the node N4 to V SS . When the potential of the succeeding output node N6 becomes low after the high level voltage of the succeeding output node N6 is transmitted to a further succeeding stage, the potential of the node N4 is low and, thus, the potential of the node N4' is still high, so that the potential of the node N4 is always clamped to the voltage V SS by the transistor Q₆. Therefore, by using the circuit of Fig. 5, it is possible to prevent the node N4 from being in an electrically floating state when the node N4 is at a low level. It is also possible to increase the reliability of operation of a shift register, because the potential of the node N4 does not fluctuate due to noise and so on. As mentioned above, according to the present invention, it is possible to constitute a shift register, for example a ring counter type shift register by using a very simple circuit structure, and to considerably reduce the width occupied by each circuit stage on a semiconductor substrate. Therefore, it becomes very advantageous to dispose the shift register parallel to a decoder as mentioned before.

Claims: A shift register having a plurality of circuit stages connected in cascade and receiving first (ø₀) and second (ø₁) clock signals different in phase from each other for driving the shift register, each of said circuit stages comprising: transfer means operatively connected to receive an input signal and said first clock signal (ø₀) including a first transistor (Q₄) having a source or drain for receiving said input signal; a second transistor (Q₅) having a gate operatively connected via said first transistor (Q₄) to receive said input signal, a source and a drain, one of said source and drain receiving said second clock signal (ø₁), said second transistor (Q₅) providing an output signal (OUT) at the other of said source and drain in response to said input signal and said second clock signal (ø₁); and a reset means (Q₆, Q₆', Q₆'') for resetting said output signal (OUT); characterised in that when said first clock signal (ø₀) is applied to said transfer means, said transfer means allows the transfer of said input signal and wherein said first clock signal (ø₀) or said transfer means is arranged such that when said first clock signal (ø₀) is not activated the gate voltage of said first transistor (Q₄) is held between Vth and 2Vth, where Vth is the threshold of said first transistor (Q4); and in that said reset means operates in response to said output signal (OUT) of a succeeding circuit stage of the shift register, the output signal of the circuit stage providing the input signal applied to the first transistor (Q₇) of the succeeding circuit stage of the shift register. A shift register according to claim 1, wherein said reset means comprises a third transistor (Q₆) which is connected in series with said second transistor (Q₅) and which is turned on and off by the output of the succeeding circuit stage. A shift register according to claim 2, wherein each of said circuit stages comprises a capacitor (C₂) connected in parallel with said third transistor (Q₆). A shift register according to claim 1, wherein said reset means comprises a third transistor (Q₆) connected in series with said second transistor (Q₅), a fourth transistor (Q₆") cross-coupled with said third transistor, and a fifth transistor (Q₆') which is connected in series with said fourth transistor (Q₆") and which is turned on and off by an output signal of the succeeding circuit stage. A shift register according to claim 4, wherein each of said circuit stages comprises a capacitor (C₂) connected to said third transistor (Q₆). A shift register according to any preceding claim, wherein the phases of said first clock signal (ø₀' ø₂) and said second clock signal (ø₁, ø₃) differ from each other approximately by 180 degrees. A shift register according to any preceding claim, wherein the voltage level of said first clock signal (ø₀, ø₂) reciprocates between a power supply voltage (Vcc) and a voltage within the range of Vth and 2Vth. A shift register according to any of claims 1 to 6, wherein said transfer means further comprises a further transistor (Q₄') which is connected in parallel with said first transistor (Q₄) and whose gate receives a bias voltage within the range of Vth to 2Vth, where Vth is the threshold voltage of said first transistor (Q₄).

Inventors: TAKEMAE, YOSHIHIRO
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: G11C19
IPC codes: G11C19