Detecting switching of access elements of phase change memory cells

1. A memory comprising:

a phase change storage element and an access element switching from a high impedance condition to a low impedance condition;

a sense amplifier couplable to said storage and access element;

a read circuit couplable to said access element to detect the switching of the access element; and

a circuit enabling the sense amplifier in response to the detection of the switching of the access element.

2. The memory according to claim 1 , wherein the access element includes a threshold switch element made of a single phase material.

3. The memory according to claim 1 , wherein the read circuit includes a peak detector.

4. A method comprising:

switching an access element from a non-threshold condition to an threshold condition in response to selection of a memory cell;

detecting the switching of the access element, and

enabling the determination of a memory cell state in response to the detection of the switching of the access element.

5. The method according to claim 4 , wherein detecting the switching of the access element includes identifying a peak rate of change of an electrical quantity.

6. The method according to claim 4 , further comprising:

providing a read biasing current, a second read current lower than the read biasing current;

applying the biasing current to the memory cell in response to the selection of the memory cell;

detecting a first steady condition of the memory cell corresponding to the first read current;

sensing a first measuring voltage in response to the detection of the first steady condition;

applying the second read current to the memory cell in response to the sensing of the first measuring voltage;

detecting a second steady condition of the memory cell corresponding to the second read current;

sensing a second measuring voltage in response to the detection of the second steady condition; and

determining the logic value stored in the memory cell according to the first and the second measuring voltages.

7. The method according to claim 6 , wherein detecting the switching of the access element and detecting the second steady condition include identifying a first change in rate of the electrical quantity, and the step of detecting the first steady condition includes identifying a second change of rate after the first change of rate.

8. The method according to claim 7 , wherein the step of detecting each change of rate includes delaying the electrical quantity by a predetermined period and identifying the corresponding change of rate when the delayed electrical quantity reaches the electrical quantity within a predetermined offset, the offsets for the first and second changes of rate having opposite signs.

9. The method according to claim 4 , wherein detecting the switching of the access element includes:

providing a derivative signal corresponding to a time-derivative of the electrical quantity, and

comparing the derivative signal with a threshold voltage.

10. The method according to claim 9 , wherein determining a logic value stored in the memory cell includes comparing the derivative signal with a further threshold voltage.

11. A system comprising:

a controller; and

a phase change memory coupled to said controller including a storage element and an access element switching from a high impedance condition to a low impedance condition, a sense amplifier couplable to said storage and access element, a read circuit couplable to said access element to detect the switching of the access element and a circuit enabling the sense amplifier in response to the detection of the switching of the access element.

12. The system according to claim 11 , wherein the access element includes a threshold switch element made of a single phase material.

13. The system according to claim 11 , wherein the read circuit includes a rate of change detector.