Patent Application
20080043513
These above recited features of the present invention will become clear from the following description, taken in conjunction with the accompanying drawings. It is to be noted, however, that the accompanying drawings illustrate only typical embodiments of the present invention and are, therefore, not to be considered limiting of the scope of the invention. The present invention may admit other equally effective embodiments.
The evaluation of the resistive state of a resistive memory cell is generally effected by means of an application of electric signals during a reading operation. During such a reading operation the resistance is sensed and a corresponding logic state, for example for a binary cell one out of “0” and “1”, is determined. The voltage regulation unit 20 applies a sensing voltage via a bit line 200 to the resistive memory cell 30. If the applied voltage at the resistive memory cell 30 remains essentially constant, the evaluation unit 10 may determine the resistive state of the resistive memory cell 30 by measuring the resulting current via a signal line 100. According to this embodiment of the present invention, a voltage regulation circuit 20 is arranged in between the evaluation unit 10 and the resistive memory cell 30. The signal line 100 is hence connected from the evaluation unit 10 to the voltage regulation circuit 20. The voltage regulation circuit 20 regulates the voltage and applies the regulated voltage via a bit line 200 to the resistive memory cell 30. For the regulation of the applied voltage the voltage regulation circuit 20 senses the actually applied voltage at the bit line 200 via a feedback line 201. Being able to determine the actual voltage via the feedback line 201, the voltage regulation circuit 20 regulates the incoming voltage from the bit line 200 and ensures that the applied voltage is maintained sufficiently constant at the resistive memory cell 30.
Since the effective electric resistance of the resistive memory cell 30 may vary dramatically, according to the respective resistive state of the resistive memory cell 30, the voltage applied to it may be subject to undesired changes. In the case that the resistive memory cell 30 is in a high resistive state, the sense voltage coming from the voltage regulation unit 20 via the bit line 200 may correspond approximately to the target voltage at the resistive memory cell, since the high resistance of the resistive memory cell 30 prevents a critical voltage drop, since only a little current is drawn from the voltage source. On the other hand, however, in the case that the resistive memory cell 30 is in a low resistive state, a substantial voltage drop may occur. In this case, according to this embodiment of the present invention, the voltage regulation circuit 20 senses the actual voltage via the feedback line 201 and regulates the voltage to a target voltage. In one embodiment, the voltage is raised to the target voltage in case a voltage drop caused a deviation from the target voltage. In one embodiment, the actual voltage at the resistive memory cell 30 is kept essentially constant corresponding to a target voltage level over the entire effective range of resistance of the resistive memory cell 30. According to one embodiment, the target voltage is in a range of ±30% of the voltage being applied in the case of the resistive memory cell 30 being in a high resistive state. According to a further embodiment, the target voltage is in a range of ±15% of the voltage being applied in the case of the resistive memory cell 30 being in a high resistive state, and, according to yet another embodiment, the target voltage is in a range of ±8% of the voltage being applied in the case of the resistive memory cell 30 being in a high resistive state.
As an example, the resistive memory cell 30 may comprise a conductive bridging element which may comprise a chalcogenide. In such a material the threshold voltage for changing the resistive state of the resistive memory cell 30 may be in a range of 200 to 250 mV. As a result, the applied voltage for evaluating the resistive state, without substantially altering the resistive state, may be well below this threshold voltage. For example, a reading voltage in a range of 100 to 150 mV may be applied to determine the resistive state of the resistive memory cell 30. In this case, the voltage regulation unit 20 may apply a sense voltage in the range of 100 mV to 150 mV to the resistive memory cell being either in a low resistive state or high resistive state. The sense voltage may drop substantially below the range of 100 mV to 150 mV where the resistive memory cell is in a low resistive state. The voltage regulation circuit 20 may then regulate the voltage to a target voltage lying in said range, or with a tolerance of ±30% in said range. The tolerance may be decreased to ±15% or to ±8%. In principal, the voltage regulation circuit 20 may raise the applied voltage to just below the threshold voltage. Variations of the actually applied sense voltage over the entire resistive state of the resistive memory cell 30 may be in a range of ±30 mV, ±15 mV and ±8 mV, according to various embodiments.
The sense voltage of the evaluation unit 12 is coupled to a voltage regulation circuit 22 via a signal line 100. The voltage regulation circuit 22 may comprise a regulation transistor 223. The regulation transistor 223 may be an n-channel FET. A gate of the regulation transistor 223 may be coupled via a line 224 to an output of an operational amplifier 221—or any other comparator circuit—which compares the applied voltage to a reference voltage 222. As being typical of such a circuitry, the operational amplifier 221 attempts to regulate the voltage being applied via the line 200 to the resistive memory cell 31 by comparing this voltage coupled to one input of the operational amplifier 221 to a reference voltage coupled to a second input of the operational amplifier 221. The latter reference voltage may be provided by a reference voltage source 222 or by the supply voltage by means of an optional voltage divider.
The resistive memory cell 31 may comprise a resistive memory element 310, comprising, for example, a chalcogenide or another solid electrolyte, or another conductive bridging material, and a selection transistor 311. The resistive memory element 310 is coupled via a bit line 200, 400 to the voltage regulation circuit 22 and to the selection transistor 311. The selection transistor 311 is furthermore coupled to a word line and a reference electrode. The resistive memory element 310 may further be arranged on the other side of the selection transistor 311, in this case the selection transistor 311 being coupled to the bit line 200, 400. Upon addressing the selection transistor 311 a current may flow from the output of the voltage regulation circuit 22 through the resistive memory element 310 and through the selection transistor 311 to a reference electrode. This current, being dependent on the applied voltage and the resistance of the resistive memory element 310, flows also through the voltage regulation circuit 22 and the evaluation unit 12. Hence, assuming that the voltage regulation circuit 22 sufficiently maintains the voltage being applied to the resistive memory cell 31, this current may be converted to an output signal of the evaluation unit 12. This output voltage then reliably corresponds to the resistive state of the resistive memory element 31.
An optional multiplexing unit 40 may be arranged in between the voltage regulation circuit 22 and a plurality of resistive memory cells 31 in order to share the evaluation unit 12 and a voltage regulation circuit 22 to more than one resistive memory cell 31. In the presence of a multiplexing unit 40, the bit line 200 may act as a master bit line and the voltage regulation circuit 22 is coupled to the multiplexing unit 40 via the master bit line 200, and is coupled to the resistive memory cell 31 via a bit line 400. In absence of the multiplexing unit 40, the voltage regulation circuit 22 is directly coupled to the memory cell 31 via a single bit line denoted by 200 and 400.
The operational amplifier 23, as shown here, may be a conventional operational amplifier and the actually shown circuitry is just one example for various known implementations and circuitries of operational amplifiers. The shown operational amplifier 23 is coupled to a supply voltage with a ground supply 230 and a supply voltage 231. One of the two inputs, here the input 232, is coupled to a reference voltage 225. The other input 233 is coupled to the voltage being applied at the point between the regulation transistor 224 and the resistive memory cell. In this way, the operational amplifier 23 regulates the applied voltage at the memory cell, by means of an appropriate control of the gate of the regulation transistor 224 via its output 234. The voltage being applied at the resistive memory cell is regulated to be equal to the reference voltage 225. An evaluation circuit may be provided which couples an output signal to a sense amplifier (SA) to determine the resistive state of the resistive element 312. The shown operational amplifier 23 acts as a differential amplifier, whose output 234 is proportional to the voltage between the two inputs 232 and 233.
The preceding description only describes advantageous exemplary embodiments of the invention. The features disclosed therein and the claims and the drawings can, therefore, be essential for the realization of the invention in its various embodiments, both individually and in any combination. While the foregoing is directed to embodiments of the present invention, other and further embodiments of this invention may be devised without departing from the basic scope of the invention, the scope of the present invention being determined by the claims that follow.
