Word line driver for DRAM embedded in a logic process
View Patent ↗A word line driver is provided for accessing a DRAM cell embedded in a conventional logic process. The DRAM cell includes a p-channel access transistor coupled to a cell capacitor. The word line driver includes an n-channel transistor located in a p-well, wherein the p-well is located in a deep n-well. The deep n-well is located in a p-type substrate. A word line couples the drain of the n-channel transistor to the gate of the p-channel access transistor. A negative boosted voltage supply applies a negative boosted voltage to the p-well and the source of the n-channel transistor. The negative boosted voltage is less than ground by an amount equal to or greater than the threshold voltage of the p-channel access transistor. The deep n-well and the p-type substrate are coupled to ground. The various polarities can be reversed in another embodiment.
1. A word line driver circuit for driving a memory cell, comprising:
a positive boosted voltage terminal configured to receive a positive boosted voltage, which is greater than a positive supply voltage by a first voltage, which is selected to minimize the sum of the sub-threshold leakage and gate-induced-drain-lowering (GIDL) leakage through a pass-gate transistor of the memory cell; and
a negative boosted voltage terminal configured to receive a negative boosted voltage equal to or more than one transistor threshold voltage more negative than a negative/ground supply voltage.
2. The word line driver circuit of claim 1 , wherein the first voltage has a value between 0.2 Volts and the absolute value of the threshold voltage of the pass-gate transistor of the memory cell.
3. The word line driver circuit of claim 1 , further comprising:
an n-channel transistor coupled between the negative boosted voltage terminal and the pass-gate transistor of the memory cell, wherein the n-channel transistor is located in a p-well region, which in turn, is located in a deep n-well region.
4. The word line driver circuit of claim 3 , further comprising:
a p-channel transistor coupled between the positive boosted voltage terminal and the pass-gate transistor of the memory cell.
5. The word line driver circuit of claim 1 , wherein the memory cell comprises a dynamic random access memory (DRAM) cell embedded in a conventional logic process.
6. The word line driver circuit of claim 5 , wherein the DRAM cell includes a capacitor having a capacitance of about 10 femto-Farads or less.
7. The word line driver circuit of claim 1 , comprising an output driver coupled to the positive boosted voltage terminal and the negative boosted voltage terminal, and having an output terminal coupled to a gate of the pass-gate transistor of the memory cell.
8. The word line driver circuit of claim 7 , further comprising a voltage translator circuit coupled to an input terminal of the output driver.
9. The word line driver circuit of claim 8 , wherein the pass-gate transistor and any transistors in the output driver and voltage translator circuit have a gate dielectric with the same thickness.
10. The word line driver circuit of claim 1 , wherein the negative boosted voltage terminal is configured to receive a negative boosted voltage more than one transistor threshold voltage more negative than the negative/ground supply voltage.
11. The word line driver circuit of claim 3 , further comprising means for biasing the deep n-well region at a voltage greater than ground.
12. The word line driver circuit of claim 3 , wherein the means for biasing the deep n-well region at a voltage greater than ground comprises the positive boosted voltage terminal.
13. The word line driver circuit of claim 3 , wherein the deep n-well region is located in a p-type substrate, wherein the p-type substrate is grounded.
14. The word line driver circuit of claim 3 , further comprises an n-channel logic transistor fabricated in a second p-well region, isolated from the p-well region.
15. The word line driver circuit of claim 14 , wherein the n-channel transistor has a gate dielectric layer that is thicker than a gate dielectric layer of the n-channel logic transistor.
16. The word line driver circuit of claim 4 , wherein the p-channel transistor is located in an n-well region, wherein the n-well region is coupled to the positive boosted voltage terminal.
17. The word line driver of claim 16 , wherein the n-well region is continuous with the deep n-well region.
18. The word line driver of claim 16 , wherein the n-well region is isolated from the deep n-well region.
19. The word line driver circuit of claim 6 , wherein the DRAM cell includes a capacitor having a capacitance of about 1.5 to 10 femto-Farads.