Devices and methods to program a memory cell

1. A method of programming a first memory cell and a second memory cell, each of the first memory cell and the second memory cell having at least a first stable state and a second stable state, the method comprising:

applying a decreasing current programming signal to the first memory cell for programming the first memory cell to one of the first and second stable states of the first memory cell;

applying a decreasing voltage programming signal to the second memory cell for programming the second memory cell to one of the first and second stable states of the second memory cell; and

detecting whether the decreasing current programming signal is less than a current threshold value or whether the decreasing voltage programming signal is less than a voltage threshold value.

2. The method of claim 1 , further comprising ceasing application of the decreasing current programming signal upon detection the decreasing current programming signal is less than the current threshold value or ceasing application of the decreasing voltage programming signal upon detection the decreasing voltage programming signal is less than the voltage threshold value.

3. The method of claim 1 , wherein the first stable state is a higher resistance state than the second stable state.

4. The method of claim 3 , wherein the one of the first and second stable states of the first memory cell is the first stable state, and wherein applying the decreasing current programming signal to the first memory cell is part of transitioning the first memory cell from the second stable state to the first stable state.

5. The method of claim 3 , wherein the one of the first and second stable states of the second memory cell is the second stable state, and wherein applying the decreasing voltage programming signal to the second memory cell is part of transitioning the second memory cell from the first stable state to the second stable state.

6. The method of claim 1 , wherein applying the decreasing current programming signal comprises applying the decreasing current programming signal after the first memory cell enters a melting phase.

7. The method of claim 6 , further comprising applying a substantially constant current programming signal to first memory cell until the first memory cell enters the melting phase.

8. The method of claim 1 , wherein applying the decreasing current programming signal comprises applying a linearly decreasing current programming signal.

9. The method of claim 1 , wherein applying the decreasing voltage programming signal comprises applying a linearly decreasing voltage programming signal.

10. The method of claim 1 , further comprising applying the decreasing current programming signal and the decreasing voltage programming signal concurrently.

11. A memory device, comprising

a memory array comprising a first memory cell and a second memory cell, wherein each of the first memory cell and the second memory cell comprises a first stable state and a second stable state; and

a memory controller configured to operate in a current mode or in a voltage mode, and to:

apply a decreasing current programming signal to the first memory cell for programming the first memory cell to one of the first and second stable states of the first memory cell,

apply a decreasing voltage programming signal to the second memory cell for programming the second memory cell to one of the first and second stable states of the second memory cell, and

detect whether the decreasing current programming signal is less than a current threshold value or whether the decreasing voltage programming signal is less than a voltage threshold value.

12. The device of claim 11 , wherein the memory controller is further configured to cease application of the decreasing current programming signal upon detection the decreasing current programming signal is less than the current threshold value or cease application of the decreasing voltage programming signal upon detection the decreasing voltage programming signal is less than the voltage threshold value.

13. The device of claim 11 , wherein the first stable state is a higher resistance state than the second stable state.

14. The device of claim 13 , wherein the one of the first and second stable states of the first memory cell is the first stable state, wherein the one of the first and second stable states of the second memory cell is the second stable state, and wherein the memory controller is further configured to:

transition the first memory cell from the second stable state to the first stable state, wherein transitioning the first memory cell from the second stable state to the first stable state comprises applying the decreasing current programming signal to the first memory cell; and

transition the second memory cell from the first stable state to the second stable state, wherein transitioning the second memory cell from the first stable state to the second stable state comprises applying the decreasing voltage programming signal to the second memory cell.

15. The device of claim 11 , wherein the memory controller is further configured to:

apply a linearly decreasing current programming signal to the first memory cell for programming the first memory cell to the one of the first and second stable states of the first memory cell; and

apply a linearly decreasing voltage programming signal to the second memory cell for programming the second memory cell to the one of the first and second stable states of the second memory cell.

16. The device of claim 11 , wherein the memory controller is further configured to apply the decreasing current programming signal to the first memory cell and to apply the decreasing voltage programming signal to the second memory cell concurrently.

17. The device of claim 11 , wherein the first memory cell and the second memory cell are phase change memory cells.

18. A system, comprising:

a memory array comprising a first memory cell and a second memory cell, wherein each of the first memory cell and the second memory cell comprises a first stable state and a second stable state;

a memory controller configured to operate in a current mode or in a voltage mode, and to:

apply a decreasing current programming signal to the first memory cell for programming the first memory cell to one of the first and second stable states of the first memory cell,

apply a decreasing voltage programming signal to the second memory cell for programming the second memory cell to one of the first and second stable states of the second memory cell, and

detect whether the decreasing current programming signal is less than a current threshold value or whether the decreasing voltage programming signal is less than a voltage threshold value; and

a processor to initiate commands to the memory controller for accessing the memory array.

19. The system of claim 18 , wherein the memory controller is further configured to cease application of the decreasing current programming signal upon detection the decreasing current programming signal is less than the current threshold value or cease application of the decreasing voltage programming signal upon detection the decreasing voltage programming signal is less than the voltage threshold value.

20. The system of claim 18 , wherein the memory controller is further configured to apply the decreasing current programming signal to the first memory cell after the first memory cell enters a melting phase.

21. The system of claim 20 , wherein the memory controller is further configured to apply a substantially constant current programming signal to the first memory cell until the first memory cell enters the melting phase.

22. The system of claim 18 , wherein the first stable state is a higher resistance state than the second stable state, wherein the one of the first and second stable states of the first memory cell is the first stable state, wherein the one of the first and second stable states of the second memory cell is the second stable state, and wherein the memory controller is further configured to:

transition the first memory cell from the second stable state to the first stable state, wherein transitioning the first memory cell from the second stable state to the first stable state comprises applying the decreasing current programming signal to the first memory cell; and

transition the second memory cell from the first stable state to the second stable state, wherein transitioning the second memory cell from the first stable state to the second stable state comprises applying the decreasing voltage programming signal to the second memory cell.

23. The system of claim 18 , wherein the memory controller is further configured to:

apply a linearly decreasing current programming signal to the first memory cell for programming the first memory cell to the one of the first and second stable states of the first memory cell; and

apply a linearly decreasing voltage programming signal to the second memory cell for programming the second memory cell to the one of the first and second stable states of the second memory cell.