Ultra dense and stable 4T SRAM cell design having NFETs and PFETs
Ultra dense and stable 4T SRAM designs are provided. In one aspect, a 4T SRAM bitcell includes: two NFETs cross-coupled with two PFETs, wherein the NFETs are both connected directly to a word line, wherein a first one of the PFETs is connected to a first bit line via a first one of the NFETs and a second one of the PFETs is connected to a second bit line via a second one of the NFETs, and wherein the PFETs are each separately connected to ground. An SRAM device including the present 4T SRAM bitcell as well as a method of operating the SRAM device are also provided.
1. A 4 transistor (4T) static random access memory (SRAM) bitcell, comprising:
two n-channel field-effect transistors (NFETs) cross-coupled with two p-channel FETs (PFETs), wherein the NFETs are both connected directly to a word line, wherein a first one of the PFETs is connected to a first bit line via a first one of the NFETs and a second one of the PFETs is connected to a second bit line via a second one of the NFETs, wherein the PFETs are each separately connected to ground, and wherein cross-coupling between the NFETs and the PFETs comprises a first connection between only two devices which are the first PFET and the first NFET, and a second connection between only two other devices which are the second PFET and the second NFET.
2. The 4T SRAM of claim 1 , wherein the NFETs and the PFETs each comprises a vertical transport field-effect transistor (VTFET).
3. The 4T SRAM of claim 2 , wherein the VTFET comprises:
at least one vertical fin channel;
a bottom source and drain at a base of the at least one vertical fin channel;
bottom spacers disposed on the bottom source and drain;
a gate along sidewalls of the at least one vertical fin channel above the bottom spacers;
top spacers above the gate at a top of the at least one vertical fin channel; and
a top source and drain above the top spacers.
4. The 4T SRAM of claim 1 , wherein the NFETs and the PFETs each comprises a fin-field-effect transistor (finFET).
5. The 4T SRAM of claim 4 , wherein the finFET comprises:
at least one fin interconnecting a source and a drain;
a gate disposed over a portion of the at least one fin that serves as a channel of the finFET; and
spacers offsetting the gate from the source and the drain.
6. The 4T SRAM of claim 1 , wherein the NFETs and the PFETs each comprises a nanosheet transistor.
7. The 4T SRAM of claim 6 , wherein the nanosheet transistor comprises:
a stack of nanosheets interconnecting a source and a drain;
a gate surrounding at least a portion of each of the nanosheets in the stack that serves as a channel of the nanosheet transistor; and
spacers offsetting the gate from the source and the drain.
8. A SRAM device, comprising:
bit lines;
word lines oriented orthogonal to the bit lines;
bitcells at an intersection of the word lines and bit lines, wherein each bit cell comprises two NFETs cross-coupled with two PFETs, wherein the NFETs are both connected directly to one of the word lines, wherein a first one of the PFETs is connected to a first one of the bit lines via a first one of the NFETs and a second one of the PFETs is connected to a second bit line via a second one of the NFETs, wherein the PFETs are each separately connected to ground, and wherein cross-coupling between the NFETs and the PFETs comprises a first connection between only two devices which are the first PFET and the first NFET, and a second connection between only two other devices which are the second PFET and the second NFET.
9. The SRAM device of claim 8 , wherein the NFETs and the PFETs each comprises a VTFET.
10. The SRAM device of claim 9 , wherein the VTFET comprises:
at least one vertical fin channel;
a bottom source and drain at a base of the at least one vertical fin channel;
bottom spacers disposed on the bottom source and drain;
a gate along sidewalls of the at least one vertical fin channel above the bottom spacers;
top spacers above the gate at a top of the at least one vertical fin channel; and
a top source and drain above the top spacers.
11. The SRAM device of claim 8 , wherein the NFETs and the PFETs each comprises a finFET.
12. The SRAM device of claim 11 , wherein the finFET comprises:
at least one fin interconnecting a source and a drain;
a gate disposed over a portion of the at least one fin that serves as a channel of the finFET; and
spacers offsetting the gate from the source and the drain.
13. The SRAM device of claim 8 , wherein the NFETs and the PFETs each comprises a nanosheet transistor.
14. The SRAM device of claim 13 , wherein the nanosheet transistor comprises:
a stack of nanosheets interconnecting a source and a drain;
a gate surrounding at least a portion of each of the nanosheets in the stack that serves as a channel of the nanosheet transistor; and
spacers offsetting the gate from the source and the drain.
15. A method of operating a SRAM device, the method comprising the steps of:
providing the SRAM device comprising:
bit lines;
word lines oriented orthogonal to the bit lines;
bitcells at an intersection of the word lines and bit lines, wherein each bit cell comprises two NFETs cross-coupled with two PFETs, wherein the NFETs are both connected directly to a given one of the word lines, wherein a first one of the PFETs is connected to a first given one of the bit lines via a first one of the NFETs forming a zero (0) node of the 4T SRAM bitcell and a second given one of the PFETs is connected to a second bit line via a second one of the NFETs forming a one (1) node of the 4T SRAM bitcell, wherein the PFETs are each separately connected to ground, and wherein cross-coupling between the NFETs and the PFETs comprises a first connection between only two devices which are the first PFET and the first NFET, and a second connection between only two other devices which are the second PFET and the second NFET;
passively retaining the zero (0) node using ground leak through the first PFET; and
actively retaining the one (1) node using the second PFET.
16. The method of claim 15 , further comprising the steps of:
applying a write pulse to the first given bit line and the second given bit line, the write pulse corresponding to a logic 0 or a logic 1 data state; and
activating the given word line to write the data state to the bit cell.
17. The method of claim 16 , wherein the write pulse is applied to the first given bit line and the second given bit line using a write driver.
18. The method of claim 16 , further comprising the steps of:
floating the first given bit line and the second given bit line;
activating the given word line; and
reading current Iread through the bitcell.
19. The method of claim 18 , wherein the Iread through the bitcell is read using a current sense amplifier.
20. The method of claim 16 , wherein the NFETs and the PFETs each comprises a VTFET, a finFET, or a nanosheet transistor.