Semiconductor memory device with memory cells operated by boosted voltage
View Patent ↗A memory using an SRAM memory cell intended for low-voltage operation is designed to decrease the threshold value of MOS transistors constituting the memory cell without substantial decrease in the static noise margin, which is the operational margin of the memory cell. To this end, a voltage Vdd′ higher than a power supply voltage Vdd of a power supply line for peripheral circuits is supplied from a power supply line for memory cells as a power supply voltage for memory cells. Since the conductance of driver MOS transistors is increased, the threshold voltage of the MOS transistors within the memory cells can be reduced without reducing the static noise margin. Further the ratio of width between the driver MOS transistor and a transfer MOS transistor can be set to 1, thereby allowing a reduction in the memory cell area.
1. A semiconductor memory device comprising:
a plurality of static memory cells each comprising N-channel first and second driver MOS transistors, N-channel first and second transfer MOS transistors whose source-drain paths are coupled between drain electrodes of the driver MOS transistors and bit lines, and P-channel first and second load MOS transistors, and being placed in an array on a semiconductor substrate,
wherein source electrodes of the driver MOS transistors are coupled to a first operating potential node, and source electrodes of the first and second load MOS transistors are coupled to a second operating potential node,
wherein a potential difference between the first operating potential node and the second operating potential node is larger than a high-level potential applied to gate electrodes of the first and second transfer MOS transistors or the bit lines,
wherein the first driver MOS transistor and the first transfer MOS transistor are formed in a first P-well region,
wherein the second driver MOS transistor and the second transfer MOS transistor are formed in a second P-well region,
wherein the first and second P-channel load MOS transistors are formed in a first N-well region, which is between the first and second P-well regions,
wherein a center line of diffusion layers of the first driver MOS transistor and the first transfer MOS transistor is parallel to a boundary between said first P-well region and said first N-well region,
wherein outer shapes of said diffusion layers are linearly symmetric against the center line, and
wherein the gate width size of the N-channel driver MOS transistors is not more than 1.4 times the gate width size of the N-channel type transfer MOS transistors.
2. The semiconductor memory device according to claim 1 ,
wherein threshold voltages of the P-channel MOS transistors are set to be of higher absolute value than threshold voltages of the N-channel MOS transistors.
3. The semiconductor memory device according to claim 1 ,
wherein a first polysilicon layer used for the gate electrode of the first transfer MOS transistor and a second polysilicon layer used for the gate of the first driver MOS transistor and the gate of the first load MOS transistor are disposed in parallel relation to each other, and a third polysilicon layer used for the gate electrode of the second transfer MOS transistor and a fourth polysilicon layer used for the gate of the second driver MOS transistor and the gate of the second load MOS transistor are disposed in parallel relation to each other.
4. The semiconductor memory device according to claim 3 ,
wherein the potential difference between the first operating potential node and the second operating potential node during a period in which the memory cells are not operating is set to be smaller than the potential difference between the first operating potential node and the second operating potential node during a period in which the memory cells are operating.
5. The semiconductor memory device according to claim 3 ,
wherein the potential of the first and second P-well regions during a period in which the memory cells are not operating is set to be smaller than the potential of the first operating potential node.
6. The semiconductor memory device according to claim 3 ,
wherein a potential of said N-well region during a period in which the memory cells are not operating is set to be larger than the potential of the second operating potential node.
7. The semiconductor memory device according to claim 3 ,
wherein the potential of said first and second P-well regions during a period in which the memory cells are operating is set to be larger than the potential of the first operating potential node.
8. The semiconductor memory device according to claim 3 ,
wherein the potential of said N-well region during a period in which the memory cells are operating is set to be smaller than the potential of the second operating potential node.
9. The semiconductor memory device according to claim 1 ,
wherein potential differences between the first operating potential node and the second operating potential node have the same value for both read operation and write operation.
10. A semiconductor memory device comprising:
a plurality of static memory cells each comprising N-channel first and second driver MOS transistors, N-channel first and second transfer MOS transistors whose source-drain paths are coupled between drain electrodes of the driver MOS transistors and bit lines, and P-channel first and second load MOS transistors, and placed in an array on a semiconductor substrate,
wherein source electrodes of the drive MOS transistors are coupled to a first operating potential node, and source electrodes of the first and second load MOS transistors are coupled to a second operating potential node,
wherein a potential difference applied between the first operating potential node and the second operating potential node is larger than a high-level potential applied to gate electrodes of the first and second transfer MOS transistors or the bit lines, and
wherein the potential difference between the first operating potential node and the second operating potential node is larger during write operation than during standby mode.
11. The semiconductor memory device according to claim 10 ,
wherein threshold voltages of the P-channel MOS transistors are set to be of higher absolute value than threshold voltages of the N-channel MOS transistors.
12. The semiconductor memory device according to claim 11 ,
wherein the first driver MOS transistor and the first transfer MOS transistor are formed in a first P-well region,
wherein the second driver MOS transistor and the second transfer MOS transistor are formed in a second P-well region,
wherein the first and second P-channel load MOS transistors are formed in a first N-well region, which is placed between said first and second P-well regions, and
wherein the gate width size of the N-channel driver MOS transistors is not more than 1.4 times the gate width size of the N-channel transfer MOS transistors.
13. The semiconductor memory device according to claim 12 ,
wherein a first center line of diffusion layers of the first driver MOS transistor and the first transfer MOS transistor is parallel to a boundary between said first P-well region and said first N-well region, and the outer shapes of said diffusion layers of the first driver MOS transistor and the first transfer MOS transistor are linearly symmetric against the first center line, and
wherein a second center line of diffusion layers of the second driver MOS transistor and the second transfer MOS transistor is parallel to the boundary between said second P-well region and said first N-well region, and the outer shapes of said diffusion layers of second driver MOS transistor and the second transfer MOS transistor are linearly symmetric against the second center line.