Voltage boosting circuits for dynamic memories

Abstract: An improved wordline boost clock circuit that can be used in high speed DRAM circuits requires only one boost capacitor (42) and discharges the wordlines faster, thus improving the DRAM access time. The basic feature of the clock circuit is in the floating gate structure of the NMOS device which drives the load to negative during the boosting. In a first embodiment of the clock (Fig. 2), the gate of a first device (24) is connected to a first node (26) through a second device (28). A second node (30), connected to a wordline, is discharged through the first (24) and a third (32) device when a third node (36) is high with a fourth node (38) low. After a sufficient discharge of the second node (30), the fourth node (38) is pulled to VDD turning the second device (28) on and a fourth device (40) off. The first (NMOS) transistor (24) has its gate and drain connected together and forms a diode. When a boost capacitor (42) pulls the first node (26) down to negative, the first device (24) stays completely off because of its diode configuration and the second node (30) is pulled to negative through the third device (32). In a second embodiment (Fig. 3), a first device (24) is connected between a boost capacitor and a second node (30). The load is discharged through a third device (32) with a fourth device (40) on but a first (24) and second device (28) off. After a sufficient discharge of the load, a fourth device (40) is turned off but a second device (28) is turned on, making the third device (32) a diode. When a fifth node (74) is pulled to ground, the second node 30 is pulled down to negative with the first device (24) on. In the second embodiment circuit the load discharges through only one NMOS device (32) and consequently discharges faster than the circuit of the first embodiment.

Description: The present invention relates to voltage boosting circuits, and more particularly to a boosting circuit for a wordline clock circuit in a semiconductor memory. Voltage boosting circuits are described in the prior art in various configurations for a variety of purposes. U.S. Patent 4,061,929, issued December 6, 1977 to Asano entitled CIRCUIT FOR OBTAINING DC VOLTAGE HIGHER THAN POWER SOURCE VOLTAGE describes a voltage boosting circuit which comprises a plurality of units connected in sequence and each composed of a condenser and a plurality of MOSFETs without any transformer or diode. U.S. Patent 4,029,973 issued June 14, 1977 to Kobayashi et al entitled VOLTAGE BOOSTER CIRCUIT USING LEVEL SHIFTER COMPOSED OF TWO COMPLEMENTARY MIS CIRCUITS discloses an improvement for a voltage booster circuit. The improvement lies in the use of MISFETs as a switching means in a level converting circuit constructed in a complementary MIS semiconductor integrated circuit and therefore the voltage loss due to the conventional switching means can be prevented. In U.S. Patent 4,216,390 issued August 5, 1980 to Stewart entitled LEVEL SHIFT CIRCUIT, a gating means, biased to pass current only during signal transitions, transfers binary signals from an input signal source to a latch circuit when the signal source and the latch are operated at similar voltages. Following data transfer, the operating voltage across the latch is increased. The voltage levels of the latch output signals are correspondingly increased but the state to which the latch was set is maintained and there is no steady state current conduction through the gating means. In U.S. Patent 4,045,691, issued August 30, 1977 to Asano, also entitled LEVEL SHIFT CIRCUIT, a level shift circuit is disclosed including an inverter connected to a first voltage supply source and supplied with an input pulse. A condenser and a directional switching element are connected in series between the output point of the inverter and one potential point of the first voltage supply source. The input of a first MOS-FET is connected to the output of the inverter while the input of a second MOS-FET is connected to a connection point between the condenser and the directional switching element. The source of the first MOS-FET is connected to a common terminal of first and second voltage supply sources while the source of the second MOS-FET is connected to the other terminal of the second voltage supply source. An output voltage is generated between a common connecting point of the drains of the first and second MOS-FETs and one potential point of the second voltage supply source. Circuits for generating pulsating potentials and voltage levels outside the range of, and/or of greater magnitude than the operating potential applied to the circuits are described in U.S. Patent 4,000,412, issued Dec. 28, 1976 to Rosenthal et al entitled VOLTAGE AMPLITUDE MULTIPLYING CIRCUITS. Each circuit includes first and second transistors for applying a first voltage to one plate of a capacitor and a second voltage to the other plate of the capacitor, during one time interval. During a subsequent time interval, the first and second transistors are turned off and a third transistor applies the second potential to the one plate of the capacitor. The change in the potential at the one plate of the capacitor is coupled to the other plate of the capacitor at which is produced n output potential outside the range of the first and second voltages. The potential difference between the first voltage and the output potential difference is greater in amplitude than the potential difference between the first and second voltages. The circuit may also include means alternately applying the first voltage and then the output potential to an output point for generating pulsating signals of greater amplitude than the magnitude of the applied potential. The outputs of two or more circuits may be combined to produce direct current (d.c.) levels. Also included are circuits which operate from a pulsating source of operating potential. The preamble of Claim 1 is based on US-A- 4,000,412. U.S. Patent 4,520,463, issued May 28, 1985 to Okumura, describes a memory circuit having an improved address decoder which is operable with a low power consumption and comprises a logic means for decoding a part of address signals provided for a plurality of address lines of a memory cell array, and a plurality of transfer gates provided between the logic means and the address lines, in which one of those transfer gates is made enabled in response to a different part of the address signals thereby to transmit the output signal of the logic means to a selected row line through the enabled transfer gate. In U.S. Patent 4,574,203, issued March 4, 1986 to Baba, a clock generating circuit is described which includes a switch control circuit controlling a C-MOS circuit including first and second transistors having first and second conductivity types, respectively. Also included in the clock generating circuit is a bootstrap capacitor having a first end connected to the junction between the first and second transistors. The switch circuit includes a third transistor, having the first conductivity type, connected between the gate of the first transistor and the junction between the first and second transistors, and a fourth transistor, having the second conductivity type, connected between the gates of the first and second transistors. The gate of the second transistor is connected to receive an input clock signal and the gates of the third and fourth transistors are connected together to receive a delayed clock signal produced by delaying the input clock signal. The second end of the bootstrap capacitor is connected to receive a further delayed and inverted clock signal. When the delayed clock signal has a first value, the switching circuit connects the gates of the first and second transistors together and an output signal with a first level is produced at the junction of the first and second transistors. When the delayed clock signal reaches a second level, the switching circuit connects the gate of the first transistor to the junction of the first and second transistors and the bootstrap capacitor boosts the output signal to a second level. Other references in the general field of voltage or level boosting include the following U.S. Patents. U.S. Patent 3,999,081 by T. Nakajima entitled CLOCK-CONTROLLED GATE CIRCUIT issued December 21, 1976. U.S. Patent 3,982,138 by Luisi et al entitled HIGH SPEED-LOW COST, CLOCK CONTROLLED CMOS LOGIC IMPLEMENTATION issued September 21, 1976. U.S. Patent 3,947,829 by Y. Suzuki entitled LOGICAL CIRCUIT APPARATUS issued March 30, 1976. U.S. Patent 3,943,377 by Y. Suzuki entitled LOGICAL CIRCUIT ARRANGEMENT EMPLOYING INSULATED GATE FIELD EFFECT TRANSISTORS issued March 9, 1976. U.S. Patent 3,852,625 by M. Kubo entitled SEMICONDUCTOR CIRCUIT issued December 3, 1974. U.S. Patent 3,801,831 by J. S. Dame entitled VOLTAGE LEVEL SHIFTING CIRCUIT issued April 2, 1974. U.S. Patent 4,129,794 entitled ELECTRICAL INTEGRATED CIRCUIT CHIPS issued December 12, 1978. U.S. Patent 4,398,100 entitled BOOSTER CIRCUIT issued August 9, 1983. U.S. Patent 4,639,622 entitled BOOSTING WORD-LINE CLOCK CIRCUIT FOR SEMICONDUCTOR MEMORY issued January 27, 1987. U.S. Patent 4,707,625 entitled SEMICONDUCTOR INTEGRATED CIRCUIT DEVICE FORMED WITH A CMOS CIRCUIT AND A BOOTSTRAP CAPACITOR issued November 17, 1987. The invention is claimed in claim 1. It provides a further improved semiconductor memory clock circuit for boosting the wordline voltage. The invention will become more apparent from the following description of preferred embodiments. FIG. 1 is a schematic illustration of a boost circuit according to the prior art. FIG. 2 is a schematic illustration of one embodiment of a boost circuit according to the present invention. FIG. 3 is a schematic illustration of another embodiment of a boost circuit according to the present invention. FIG. 4 is a schematic illustration of the boost circuit of FIG. 2 incorporated into a CMOS DRAM system. FIGS. 5, 6 and 7 are curves illustrating the operation of the system of FIG. 4. FIG. 8 is a schematic illustration of the boost circuit of FIG. 3 incorporated into a CMOS DRAM system. FIGS. 9, 10 and 11 are curves illustrating the operation of the system of FIG. 8. Described are new wordline boost clocks that can preferably be used in MOS DRAMs. These new circuits require only one boost capacitor and discharge the wordlines faster, improving the DRAM access time. In conventional CMOS DRAM designs, the wordlines are frequently boosted above the VDD power supply level for an n-array or below VDD for a p-array to restore the full signal to the cells. A high speed CMOS DRAM is disclosed wherein a p-array is used with a negative substrate bias. The wordlines are boosted below GND (ground potential) at least bV the magnitude of the threshold voltage of the array transfer device to restore the full signal to the cells. The negative substrate bias makes NMOS devices handle negative voltages during the boosting phase of the wordline clock. The proposed boost wordline clock circuits demonstrate significant advantages over the prior art circuits. One basic difference between the new circuits of the present invention and the prior art, for example the design described in U.S. Patent 4,678,941, is in the methods of floating the gate of the NMOS device which drives the load to negative during the boosting. FIGS. 1, 2 and 3 explain the difference in the basic concepts with simplified schematic diagrams of the designs. In FIG. 1, the prior art design, the load (wordlines, node 2) is discharged to ground through devices 4 and 8 with device 6 off. After the load is sufficiently discharged, the gate of device 8 is pulled down to |V TP | of device 10 as node 12 goes to ground. Then the boost capacitor 14 further pulls down the gate to negative when node 16 goes to ground. With the gate of device 8 off, the node 18 is isolated from the rest of the circuit except device 4 and it floats. The boost capacitor 20 pulls the load to negative as node 22 goes to ground. In FIGS. 2 and 3, simplified embodiments of the circuits of the present invention are shown. In FIG. 2, the gate of device 24 is connected to the node 26 through device 28. The node 30 (wordline load) is discharged through device 32 and device 24 when node 36 is high with node 38 low. After a sufficient discharge of node 30, node 38 is pulled to VDD turning device 28 on and device 40 off. The NMOS transistor 24 has its gate and drain connected together and forms a diode. When the boost capacitor 42 pulls the node 26 down to negative, device 24 stays completely off because of its diode configuration and node 30 is pulled to negative through device 32. In FIG. 3, device 32 is connected between the boost capacitor 42 and the node 30. The wordline load at node 30 is discharged through device 24 with device 40 on but device 28 and device 32 off. After a sufficient discharge of the load, device 40 is turned off but device 28 is turned on, making device 24 a diode. When node 74 is pulled to ground, the node 30 is pulled down to negative with device 32 on. Unlike the circuits of FIG. 1 and FIG. 2, in FIG. 3, the load discharges through only one NMOS device and consequently discharges faster than the other two circuits. FIG. 4 shows the circuit of FIG. 2 incorporated into a boost clock system, while FIGS. 5, 6 and 7 show the waveforms of the various nodes. A capacitive load is connected to node 30. The circuit operates as follows. During standby: Initially, timing signals on leads 106 and 108 connected to the input of NOR gate 58 are high and the output of NOR gate 58 connected to node 36 is low. Device 34 and device 32 whose gates are connected to node 36 are on and off, respectively and the load is maintained at VDD. Node 36 is connected to the NAND gate 60 in two ways, i.e., directly and after going through the inverter chain composed of inverters 62, 64, 66 and 68. The output node 70 of gate 60 is high and node 38 is low. Device 40 is on and device 28 is off and the node 72 is at VDD with device 24 on. Node 26 is at GND but device 32 is off, preventing the load from discharging to the ground. Node 74 is high and has the opposite polarity of node 38 because of the delay chain composed of inverters 78, 80, 82, 84, 86, 88 and 90. The NMOS transistor 92 is on with device 94 off, keeping the node 96 at VDD-V TN . The NAND gate 98 has nodes 36 and 74 as input and its output node 100 is high, keeping device 102 off. The boost capacitor 42, which is connected between nodes 96 and 26, is an inversion capacitor. It has a voltage drop of VDD-V TN across its gate and source/drain and is fully on with the maximum capacitance. During discharging and boosting: When the timing signals on leads 106 and 108 fall to GND, the node 36 goes high, turning device 32 on while turning device 34 off. The NAND gate 60 output node 70 stays high until node 110 goes high after the time delay determined by the inverters 62 through 68. This keeps node 72 high, discharging the load to ground through devices 32 and 24. The time delay determines the discharging time of the load and is easily changed by adding an even number of inverters between inverter 68 and gate 60. The high state at node 36 will immediately make node 100 go low, turning device 102 on slowly because of the relatively small W/L ratio of gate 98 compared to the size of device 102. This will pull the node 96 up from VDD-V TN to VDD slowly. The discharging of the load through device 32 and device 24 causes the node 26 to bump up. This in turn bumps the node 96 up because of the capacitive coupling through capacitor 42. However, the bump is smaller than V TN and the node 96 stays below VDD. If the node 96 were precharged to VDD instead of VDD-V TN , the bump would have caused the node 96 to overshoot too high above VDD. After the bump in node 26 disappears, the node 96 is fully charged to VDD. After the time delay determined by the inverters 62 through 68, node 70 goes low. This turns device 28 on but turns device 40 off. Device 24 is now in a diode configuration. The low state at node 70 ripples through the inverter chain 78 through 90 and forces node 74 to low. This turns on device 94 at the expense of devices 92 and 102. Node 96 is discharged through device 94 from VDD and GND and consequently, the node 26 is boosted to negative from GND through the boost capacitor 42. With device 32 on, the node 30 is boosted to the same voltage as node 26. During resetting: Resetting is accomplished by pulling either the timing signal on lead 106 or 108 to VDD. No deleterious overshoot or undershoot is observed during resetting. FIG. 8 shows the the circuit of FIG. 3 incorporated into the boost clock system while FIGS. 9, 10 and 11 show the waveforms of the various nodes. A capacitive load is connected to the node 30. The circuit operates as follows. During standby: Initially, the timing signals on leads 106 and 108 are high and the output of the NOR gate 58 connected to node 36 is low. This in turn makes node 26 high with device 40 off. Device 34 whose gate is connected to node 36 is on and pulls the node 30 to VDD, which causes device 94 and device 124 to discharge nodes 72 and 126-to ground. The output node 70 of the NAND gate 60 is high. This forces node 74 to be high and node 128 to be low. Consequently, devices 28 and 32 are off. The high state at node 30 forces node 130 to be low. The boost capacitor 42, which is connected between node 74 and 130, is an inversion capacitor. It has a voltage drop of VDD across its gate and source/drain and is fully on with the maximum capcitance. During discharging and boosting: When the timing signals on leads 106 and 108 fall to GND, the node 36 goes high and turns device 34 off. The NAND gate output node 70 stays high until node 132 goes high after the time delay determined by the six inverters 134, 136, 138, 140, 142 and 144. As long as node 70 stays high, node 26 is low and pulls node 72 high, turning device 24 on. The W/L of device 94 is chosen such that device 40 could pull node 72 to high with device 94 fully on. Device 24 completely discharges node 30 and also turns both devices 124 and 94 off. After the time delay determined by the inverters 134 through 144, node 70 goes low and forces node 26 to high. This shuts device 40 off but turns device 150 on. This connects the gate and drain of device 152 together, forming a diode configuration. Nodes 126 and 72 are connected together and discharge to V TN of device 152. The low state at node 70 ripples through the inverter chain 160, 162, 164, 166, 168, 170, 172 and 174. After a five inverter delay, node 176 connected after inverter 168 goes high which turns device 32 and device 28 on after going through NAND gate 178 and inverter 180. With device 28 on, device 24 is in a diode configuration. The high state at node 176 pulls node 74 to ground after three inverter delay, which boosts node 130 to negative through the boosting capacitor 42. Since device 32 is on, node 30 is boosted to the same voltage as node 130. During resetting: Resetting is accomplished by pulling either the timing signal on lead 106 or 108 to VDD. Node 36 becomes low and device 34 turns on, pulling node 30 high. Gate 178 has node 36 as one of its input and turns devices 28 and device 32 off by pulling node 128 to low. Without the NAND gate 178, node 128 would have stayed at VDD and would have an unacceptable overshoot due to the capacitive coupling through device 32 when node 30 is pulled to VDD. No deleterious overshoot or undershoot is observed during resetting.

Claims: A voltage boosting clock circuit for semiconductor memory array wordlines comprising: a charging means (34) for charging a capacitive load to a first potential level connected between a source of potential (VDD) and the capacitive load in a first cycle, a discharging means (24) for discharging the capacitive load to the ground potential level (GND) connected between the capacitive load and a source of ground potential (GND), a boosting means (32, 42) for boosting the capacitive load to a second potential level, said boosting means (32, 42) comprising a switching means (32) connected between a boosting capacitor means (42) and the capacitive load, whereby a first main electrode of the switching means (32) is connected to the capacitive load, a second main electrode thereof is connected to a first electrode of the boosting capacitor means (42) and a control electrode thereof is connected to a source of timing signals, a second electrode of the boosting capacitor means (42) being connected to the source of timing signals; characterized in that the capacitive load is boosted to the second potential level in a third cycle after the capacitive load has been discharged to or nearly to ground potential (GND) in a second cycle. The voltage boosting clock circuit according to claim 1 characterized in that the second potential level is more negative than the ground potential level (GND). The voltage boosting clock circuit according to any one of the above claims characterized in that the charging means (34) comprises a first transistor (34) where a first main electrode is connected to the source of potential (VDD), a second main electrode is connected to the capacitive load and a control electrode is connected to the source of timing signals. The voltage boosting clock circuit according to any one of the above claims characterized in that the switching means (32) is a second transistor (32). The voltage boosting clock circuit according to any one of the above claims characterized in that in response of the timing signals, the discharging means (24) forms a diode between the capacitive load and the source of ground potential (GND). The voltage boosting clock circuit according to any one of the above claims characterized in that the discharging means (24) comprises a third transistor (24), a fourth transistor (28) and a fifth transistor (40), where a first main electrode of the third transistor (24) is connected to the capacitive load and a second main electrode thereof is connected to the source of ground potential (GND), a first main electrode of the fourth transistor (28) is connected to the first main electrode of the third transistor (24), a first main electrode of the fifth transistor (40) is connected to the source of potential (VDD), a second main electrode of the fourth (28) and a second main electrode of the fifth transistor (40) are connected to a control electrode of the third transistor (24), a control electrode of the fourth (28) and a control electrode of the fifth transistor (40) are connected to the source of timing signals, wherein in response of the timing signals, the third transistor (24) is switchable completely off by means of the fourth transistor (28) (fig.2). The voltage boosting clock circuit according to any one of the above claims characterized in that the source of timing signals in a first level condition causes the charging means (34) to charge the capacitive load to the first potential level, and in response to a second level condition of the source of timing signals the discharging means (24) discharges the capacitive load to the ground potential level (GND) and the boosting means (32, 42) boosts the capacitive load to the second potential level after the capacitive load has been sufficiently discharged. The voltage boosting clock circuit according to any one of the above claims characterized in that the capacitive load is dischargable through a single transistor (24) (Fig. 3). The voltage boosting clock circuit according to claims 1 to 7 characterized by further comprising: a first logic gate (60) having an output terminal connected to the control electrode of the fifth transistor (40) and having a first input terminal connected to the source of timing signals at a second node (36), a first inverter delay circuit (62, 64, 66, 68) having an input terminal connected to the source of timing signals at the second node (36) and an output terminal (110) connected to a second input terminal of the first logic gate (60), a second logic gate (98) having a first input terminal connected to the source of timing signals at the second node (36), a second inverter delay circuit (78, 80, 82, 84, 86, 88, 90) having an input terminal connected to the output terminal of the first logic gate (60) having a first output terminal connected to a second input terminal of the second logic gate (98), and having a second output terminal connected to the boost capacitor element (42) (Fig. 4). The voltage boosting clock circuit according to claim 9 characterized in that the timing signals at the second node (36) are at the first level condition, the first transistor (34) having the control electrode connected to the second node (36) is initially on, placing the capacitive load at the first potential level and the second transistor (32) is initially off, placing a third node (26) at the ground potential level (GND), the fifth transistor (40) is initially on and the fourth transistor (28) is initially off, the third transistor (24) is on, having the control electrode connected to the source of potential (VDD) through the on fifth transistor (40), and wherein, in response to the timing signals at the second node (36) going to the second level condition, the first transistor (34) turns off, the second transistor (32) goes on, the timing signal second level condition on the second node (36) is inverted by the first inverter delay circuit (62-68) and maintains the fifth transistor (40) on, whereby the third transistor (24) stays on and the first potential level on the capacitive load is discharged through the second (32) and third (24) transistors to the ground potential level (GND) (Fig. 4). The voltage boosting circuit according to any one of claims 9 to 10 characterized by further comprising: a sixth transistor (102) having a control electrode connected to the output of the second logic gate (98), a first main electrode connected to the boost capacitor means (42) and a second main electrode connected to the source of potential (VDD), a seventh transistor (94) having a control electrode connected to a second output terminal of the second inverter delay circuit (78-90), a first main electrode connected to the first main electrode of the sixth transistor (102) and a second main electrode connected to the source of ground potential (GND), an eighth transistor (92) having a first main electrode connected to the source of potential (VDD), a control electrode connected the first output terminal node (74) of the second inverter delay circuit (78-90) and to the second input terminal of the second logic gate (98) and a second main electrode connected to the first main electrodes of the sixth (102) and seventh (94) transistors and to the boost capacitor means (42), wherein the sixth transistor (102) turns on in response to the second signal timing signals at the second node (36) connected through the second logic gate (98), wherein the seventh transistor (94) turning on in response to the output from the second inverter delay circuit (78-90) hereby discharging the boost capacitor means (42) to the ground potential level (GND), whereby the third node (26) and the capacitive load to be at the second potential level (Fig. 4). The voltage boosting clock circuit according to claims 1 to 8 characterized by further comprising: a first logic gate (60) and a second logic gate (178), a third logic gate (120) having an output terminal connected to the control electrode of the fifth transistor (40) and having a first input terminal connected to the source of timing signals at the second node (36), the first logic gate (60) having an output terminal connected to a second input terminal of the third logic gate (120) and a first input terminal connected to the source of timing signals at the second node (36), a first inverter delay circuit (134-144) having an input terminal connected to the source of timing signals at the second node (36) and an output terminal connected to a second input terminal of the first logic gate (60), the second logic gate (178) having an output connected to the control electrode of the third transistor (24) and having a first input terminal connected to the timing signals at the second node (36), a second inverter delay circuit (160-174) having an input terminal connected to the output terminal of the first logic gate (60) having a first output terminal (176) connected to a second input terminal of the second logic gate (178) and having second output terminal (74) connected to the boost capacitor means (42) (Fig. 8). The voltage boosting clock circuit according to claim 12 characterized in that the timing signals at the second node (36) are at the first level condition, the first transistor (34) having the control electrode connected to the second node (36) is initially on, placing the capacitive load at the first potential level and the third transistor (24) and the fourth transistor (28) are initially off, the fifth transistor (40) is initially off and the fourth transistor (28) is initially off and the fifth transistor (40) is on, and wherein, in response to the timing signals at the second node (36) going to the second level condition, the first transistor (34) turns off, the second transistor (32) goes on and discharges the capacitive load, the timing signal second level condition on the second node (36) is inverted by the first inverter delay circuit (134-144) and turns the fifth transistor (40) off, whereby the fourth transistor (28) goes on and the potential on the capacitive load goes to the second potential level through the second transistor (32) (fig.8). The voltage boosting circuit according to any one of the above claims characterized in that the first (34) and fifth (40) transistors are PMOS transistor types and the second (32), third (24) and fourth (28) transistors are NMOS transistor types.

Inventors: DHONG SANG HOO, HWANG WEI, LU NICKY CHAU-CHUN, DHONG, SANG HOO, HWANG, WEI, LU, NICKY CHAU-CHUN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11C11
IPC codes: G11C11, H03K5