Read, write, and erase circuit for programmable memory devices
View Patent ↗A circuit for writing, reading, and erasing a programmable device is disclosed. The programmable device includes an ion conductor and a plurality of electrodes. Electrical properties of the device are altered by applying a sufficient bias across the electrode to form a conductive region within the ion conductor. The circuit can be used to program and read multiple bits within a single programmable device.
1. A method of programming a programmable variable resistance cell, said method comprising:
forcing a programming current into the programmable variable resistance cell for a predetermined period of time, wherein the programming current instigates a decrease of a resistance in the programmable variable resistance cell;
allowing said programmable variable resistance cell to limit a voltage across said variable resistance cell to a threshold voltage for programming of the programmable variable resistance cell when said programming current is forced into said programmable variable resistance cell, wherein a programmed resistance of the programmable variable resistance cell is proportional to said threshold voltage divided by said programming current; and
generating said programming current from a quantized number of current sources, the quantized number corresponds to a number of resistance states of the programmable variable resistance cell.
2. The method as recited in claim 1 , wherein forcing a programming current into the programmable variable resistance cell reduces the resistance from one programmed state to a lower resistance of a lower programmed state without starting from a full erase state.
3. A method of sensing a resistance of a programmable variable resistance cell, said method comprising:
forcing a read bias across the programmable variable resistance cell to form a generated read current;
measuring the generated read current by said read bias across said programmable variable resistance cell;
comparing said generated read current to a programming current reduced by the ratio of said read bias and a threshold voltage for programming said programmable variable resistance cell; and
generating a digital output from said comparing step, the digital output corresponding to said resistance state sensed.
4. The method as recited in claim 3 , wherein said comparing step comprises:
comparing the generated read current to a quantized number of programming currents reduced by the ratio of said read bias and said threshold voltage to an output digital value corresponding to the current level of said generated read current; and
converting said digital value to a binary output corresponding to said digital value.
5. A method of erasing a programmable variable resistance cell, said method comprising:
forcing an erase current from a constant current source into the programmable variable resistance cell for a predetermined period of time, wherein the erase current instigates an increase of resistance in the programmable variable resistance cell; and
generating said erase current by a quantized number of constant current sources, wherein said quantized number corresponds to a number of resistance states available to be stored in the programmable variable resistance cell.
6. The method as recited in claim 5 , wherein forcing an erase current into the programmable variable resistance cell for a predetermined period of time increases the resistance of the programmable variable resistance cell to a higher resistance value, wherein the resistance is not a resistance of a complete erase of the programmable variable resistance cell.
7. A memory circuit, comprising:
an array of programmable variable resistance cells, wherein each programmable variable resistance cell comprises an anode and a cathode, wherein, when a programming bias is applied across said anode and said cathode in a first direction a reduction in resistance in said programmable variable resistance cell is obtained, and an increase in resistance is obtained when a voltage threshold for an erase is developed in a second direction;
said array of programmable variable resistance cells includes a plurality of variable resistance cells coupled together with a common anode, wherein said plurality of programmable variable resistance cells are isolated by isolation switches on an uncommon terminal;
a first distinct number of current sources switchably connected to the plurality of programmable variable resistance cells through said isolation switches for forcing a programming current into at least one of the plurality of programmable variable resistance cells;
a second distinct number of current sources switchably connected to the plurality of programmable variable resistance cells through said isolation switches for forcing an erase current into at least one of the plurality of programmable variable resistance cells;
a switch connected to a supply voltage to force a negative voltage across at least one of the plurality of programmable variable resistance cells;
a circuit to force a read bias across at least one of the plurality of programmable variable resistance cells to form a produced current;
a measurement circuit to measure the produced current to form a measured current;
a current comparison circuit to compare the measured current to one or more reference currents to form a compared value;
a circuit to convert the compared value to digital output; and
a circuit to convert the digital output to a binary output.
8. The memory circuit of claim 7 , wherein said array of programmable variable resistance cells includes a plurality of variable resistance cells coupled together with a common cathode.
9. The memory circuit of claim 7 , wherein said isolation switches, current sources, measurement circuit, current comparison circuit, circuit to convert, and circuit to force a read bias are implemented with transistors.
10. The memory circuit of claim 7 , wherein said circuit consists of one current source and one reference current.
11. The memory circuit of claim 7 , wherein said circuit to force a read bias across at least one of the plurality of programmable variable resistance cells and said current measurement circuit comprise an operational amplifier including a positive input coupled to a voltage reference, a negative input coupled to the output of a regulation transistor, which is coupled to said memory array, and an output coupled to a gate of said regulation transistor, and a third terminal of said regulation transistor is connected to a diode connected transistor.
12. A circuit for generating related programming currents and read references currents comprising:
a common bias source coupled to a plurality of transconductance devices;
a discrete number of transconductance devices used as current sources for producing districted programming currents having current values dependent on a ratio of a size of the transconductance devices; and
a discrete number of transconductance devices used as current sources to produce read reference currents, each of said discrete number of transconductance devices having a size proportional to a ratio of a read bias and a threshold voltage for programming said programmable variable resistance cell.
13. The memory circuit of claim 12 , wherein said transconductance devices are implemented with transistors.
14. The memory circuit of claim 12 , wherein said ratio of said read bias and said threshold voltage for programming said programmable variable resistance cell is 1:2.