Resistive memory with low voltage operation

1. A memory device, comprising:

an array of Resistive Random Access Memory (RRAM) memory cells arranged in rows and columns;

first column select transistors connected to a source line of each RRAM memory cell; and

second column select transistors connected to a bit line of each RRAM memory cell,

wherein each RRAM memory cell includes a RRAM resistive element connected directly between the source line and the bit line.

2. The memory device of claim 1 , further comprising:

access transistors of the RRAM memory cells that connect between the source line and the bit line of each RRAM memory cell; and

metal wiring connected across source/drain terminals of the access transistors, wherein the metal wiring bypasses the access transistors to directly connect the RRAM resistive element of each RRAM memory cell between the source line and the bit line.

3. The memory device of claim 1 , wherein there is no access transistor between the source line and the bit line for each RRAM memory cell.

4. The memory device of claim 1 , further comprising a regulated write voltage circuit comprising:

an operational amplifier configured to receive a reference write voltage signal at a first input terminal; and

a write drive transistor including a gate terminal connected to an output of the operational amplifier, a first S/D terminal configured to receive a power supply voltage, and a second S/D terminal connected, via a node, to a second input terminal of the operational amplifier to form a positive feedback loop, wherein the node is operably connected to the bit line and the source line of a RRAM memory cell by a write select transistor.

5. The memory device of claim 1 , further comprising a low voltage current limiter circuit comprising:

an operational amplifier configured to receive a reference voltage signal at a first input terminal;

a first transistor including a first gate terminal connected to an output of the operational amplifier, a first S/D terminal connected to a current mirror circuit, and a second S/D terminal connected, via a first node, to a second input terminal of the op operational amplifier to form a negative feedback loop; and

a second transistor including a third S/D terminal connected to the first node, a fourth S/D terminal connected to ground, and a second gate terminal connected, via a second node, to a third gate terminal of a bottom transistor of the array of RRAM memory cells,

wherein the second node is connected to the current mirror circuit,

wherein the output of the op amp is connected to a fourth gate terminal of a source line select transistor connected in series with the bottom transistor, and

wherein the low voltage current limiter circuit configured to control the source line select transistor and the bottom transistor to prevent over-SET of a selected RRAM memory cell.

6. The memory device of claim 1 , further comprising a write termination circuit comprising:

a first transistor and a second transistor connected in series with a node therebetween to connect respective drain terminals, wherein a first gate terminal of the first transistor is connected to a regulated write voltage circuit for generating a reference write current; and

a buffer including an input to connect to the node and an output connected to a second gate terminal of a write select transistor of the array of RRAM memory cells, wherein the write termination circuit is configured to provide a write disable signal to the write select transistor in response to detecting a threshold value of a write operation to prevent over-SET/RESET.

7. The memory device of claim 1 , further comprising a low read voltage generator circuit connected to a gate of a clamping transistor of a RRAM memory cell, the low read voltage generator circuit configured to control the clamping transistor to define a read voltage for the RRAM memory cell to avoid read disturb during a read operation with a sense amplifier circuit having process, voltage, and temperature (PVT) tracking.

8. The memory device of claim 1 , wherein:

a word line and associated write circuitry is removed from the memory device.

9. The memory device of claim 1 , further comprising a metal wiring connected across source/drain terminals of access transistors of the RRAM memory cells.

10. A method of reducing operating voltage of a memory device, comprising:

providing a memory array comprising Resistive Random Access Memory (RRAM) memory cells;

controlling a plurality of column select transistors to activate a source line and a bit line of a selected RRAM memory cell; and

programming a RRAM resistive element of the selected RRAM memory cell with a write current, wherein the RRAM resistive element is connected directly between the source line and the bit line.

11. The method of claim 10 , further comprising:

providing a metal wiring across source/drain terminals of an access transistor of each RRAM memory cell, wherein the metal wiring bypasses the access transistor to directly connect the RRAM resistive element between the source line and the bit line.

12. The method of claim 10 , further comprising:

removing an access transistor between the source and the bit line.

13. The method of claim 10 , further comprising:

providing a reference write voltage signal at a first input terminal of an operational amplifier; and

generating a write voltage with a write drive transistor arranged in a positive feedback loop with the operational amplifier.

14. The method of claim 13 , further comprising:

activating a write select transistor to provide the write current to the selected RRAM cell based on the write voltage.

15. The method of claim 14 , further comprising:

providing a reference write current based on the reference write voltage signal;

detecting that a threshold value is reached during a write to the selected RRAM memory cell based on the reference write current; and

applying a write termination signal to the write select transistor in response to determining that the threshold value is reached to prevent over-SET of the selected RRAM memory cell.

16. The method of claim 13 , further comprising:

providing a reference current with a current mirror circuit;

providing a reference write voltage signal at a first input terminal of an operational amplifier;

generating an output voltage with a first transistor arranged in a negative feedback loop with the operational amplifier;

generating a gate voltage with a second transistor connected in series with the first transistor and the current mirror circuit; and

applying the output voltage and the gate voltage respectively to a source line select transistor and a bottom transistor to limit the write current and prevent over-SET/RESET of the selected RRAM memory cell.

17. The method of claim 10 , further comprising:

controlling a gate terminal of a clamping transistor of a RRAM memory cell to define a read voltage for the RRAM memory cell, and to avoid read disturb during a read operation with a sense amplifier circuit having process, voltage, and temperature (PVT) tracking.

18. A memory device, comprising:

an array of Resistive Random Access Memory (RRAM) memory cells arranged in rows and columns, each RRAM memory cell comprising:

a resistive RRAM element including a first terminal connected to a bit line; and

a plurality of access transistors arranged in a parallel structure and including first S/D terminals connected to a second terminal of the resistive RRAM element, second S/D terminals connected to a source line, and gate terminals connected to a word line, wherein the parallel structure of the plurality of access transistors reduces IR drop of a write current applied to the resistive RRAM element, and wherein each RRAM memory cell includes a RRAM resistive element connected directly between the source line and the bit line.

19. The memory device of claim 18 , wherein the parallel structure of the plurality of access transistors is disposed between the resistive RRAM element and the source line.

20. The memory device of claim 18 , wherein gate terminals of the plurality of access transistors are connected together.