# Patent

## Title
Devices and methods for writing to a memory cell of a memory

## Abstract

A method for writing to a memory cell of a memory is disclosed. The method includes generating a write voltage, generating a write current that flows from one of a bit line and a source line of the memory, generating a mirror current that mirrors the write current, generating a compliance current, generating a write detect voltage based on the mirror current and the compliance current, detecting the write detect voltage, when it is detected that the write detect voltage is less or greater than a threshold value, permitting application of the write voltage to the memory cell of the memory, and when it is detected that the write detect voltage increases or decreases to the threshold value, inhibiting the application of the write voltage to the memory cell of the memory. A device that performs the method is also disclosed.

## Background
BACKGROUND
A resistive random access memory (RRAM) includes a plurality of RRAM cells that are operable to transition from a high resistance state to a low resistance state to thereby “set” the RRAM cells, and conversely to “reset” the RRAM cells. The high resistance state and the low resistance state represent a single bit “0” or “1” of data, whereby data are stored in the RRAM. The RRAM cells are set or reset by applying different voltages and currents on the RRAM cells. These voltages and currents cause stress on the RRAM cells when applied on the RRAM cells for a prolonged period of time.



BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
FIG. 1 is a schematic diagram of an exemplary device for writing to a memory cell of a memory in accordance with some embodiments.
FIG. 2 is a schematic diagram illustrating a voltage-generating circuit of a device in accordance with some embodiments.
FIG. 3 is a schematic diagram illustrating a device connected to a memory in accordance with some embodiments.
FIG. 4 is a schematic diagram illustrating a memory cell of a memory in accordance with some embodiments.
FIG. 5 is a schematic diagram illustrating a device connected to a memory cell of a memory in accordance with some embodiments.
FIG. 6 is a flowchart of an exemplary method for writing to a memory cell of a memory in accordance with some embodiments.
FIG. 7 is a schematic diagram illustrating a device connected to a memory cell of a memory in accordance with some embodiments.
FIG. 8 is flowchart of another exemplary method for writing to a memory cell of a memory in accordance with some embodiments.
FIG. 9 is a schematic diagram of another exemplary device for writing to a memory cell of a memory in accordance with some embodiments.



DETAILED DESCRIPTION
Before the present disclosure is described in greater detail, it should be noted that like elements are denoted by the same reference numerals throughout the disclosure.
The present disclosure provides a method for writing to, i.e., setting or resetting, a memory cell of a memory using a device. In an exemplary method, the device generates a write voltage, permits application of the write voltage to the memory cell, and generates a write current that flows through the memory cell to set or reset the memory cell. When the memory cell is set or reset, the device inhibits the application of the write voltage to the memory cell, thereby stopping flow of the write current through the memory cell.
FIG. 1 is a schematic diagram of an exemplary device 100 for writing to a memory cell of a memory in accordance with some embodiments.
As illustrated in FIG. 1, the device 100 includes a current mirror circuit 110, a voltage-generating circuit 120, a current source circuit 130, and a switch circuit 140.
The current mirror circuit 110 includes a first transistor 110a and a second transistor 110b. In this exemplary embodiment, each of the first and second transistors 110a, 110b is a P-type metal-oxide-semiconductor (PMOS) transistor and includes a source terminal, a drain terminal, and a gate terminal. The gate and drain terminals of the first transistor 110a are connected to each other and to the gate terminal of the second transistor 110b. The drain terminal of the second transistor 110b is connected to a node 150.
Since the gate and drain terminals of the first transistor 110a are connected, the first transistor 110a is configured as a diode-connected transistor, and thus operates in a saturation region. Further, since the gate terminal of the second transistor 110b is connected to the gate terminal of the first transistor 110a, a voltage across the gate terminal and the source terminal, i.e., a gate-source voltage, of the second transistor 110b is substantially equal to a gate-source voltage of the first transistor 110a. As such, the second transistor 110b is configured to mirror a write current Iwrite flowing through the first transistor 110a. 
In an alternative exemplary embodiment, at least one of the first and second transistors 110a, 110b of the current mirror circuit 110 is an N-type metal-oxide-semiconductor (NMOS) transistor.
FIG. 2 is a schematic diagram illustrating the voltage-generating circuit 120 of the device 100 in accordance with some embodiments.
As illustrated in FIG. 2, the voltage-generating circuit 120 includes a first transistor 210, a second transistor 220, a resistor 230, and an operational amplifier 240. In this exemplary embodiment, each of the first and second transistors 210, 220 is a PMOS transistor. The first and second transistors 210, 220 and the resistor 230 are connected in series between a supply voltage VDD and the ground. In particular, the first transistor 210 includes a source terminal connected to the supply voltage VDD, a drain terminal connected to a node 250, and a gate terminal. The second transistor 220 is configured as a diode-connected transistor, and includes a source terminal connected to the node 250, and a drain terminal and a gate terminal connected to each other and to a node 260. The resistor 230 is connected between the node 260 and the ground.
The operational amplifier 240 has an inverting input terminal, a non-inverting input terminal connected to the node 260, and an output terminal connected to the gate terminal of the first transistor 210.
In operation, when a first input voltage Vi1 at the node 260 is initially greater than a second input voltage Vi2 at the inverting input terminal of the operational amplifier 240, an output Vo at the output terminal of the operational amplifier 240 is high and the first transistor 210 is thus in a low conduction state. This causes the first input voltage Vi1 to decrease. When the first input voltage Vi1 decreases to less than the second input voltage Vi2, the output Vo goes low. This causes the first transistor 210 to transition from the low conduction state to a high conduction state. This, in turn, causes the first input voltage Vi1 to increase. When the first input voltage Vi1 increases to greater than the second input voltage Vi2, the output Vo goes high again. This causes the first transistor 210 to transition from the high conduction state back to the low conduction state. This, in turn, causes the first input voltage Vi1 to decrease again.
Based on the operation of the voltage-generating circuit 120, the voltage-generating circuit 120 is configured to generate a write voltage Vch at the node 250 thereof substantially equal to the sum of the second input voltage Vi2 and a threshold voltage of the second transistor 220.
In an alternative exemplary embodiment, at least one of the first and second transistors 210, 220 of the voltage-generating circuit 120 is an NMOS transistor.
Referring back to FIG. 1, the current source circuit 130 is configured to generate a predetermined compliance current Icompliance. In this exemplary embodiment, the current source circuit 130 includes a constant current source 130a connected between the node 150 and the ground. In an exemplary embodiment, when a mirror current Imirror flowing through the second transistor 110b is less/greater than the predetermined compliance current Icompliance, a write detect voltage Vwd less/greater than a threshold value, e.g., half the write voltage Vch, is generated at the node 150. On the other hand, in an exemplary embodiment, when a mirror current Imirror flowing through the second transistor 110b is substantially equal to the predetermined compliance current Icompliance, a write detect voltage Vwd substantially equal to a threshold value, e.g., half the write voltage Vch, is generated at the node 150.
The switch circuit 140 includes a logic circuit module 140a and a transmission gate module 140b. 
The logic circuit module 140a is configured to detect the write detect voltage Vwd, and to generate a pair of logic levels indicative of the write detect voltage Vwd detected thereby. In this exemplary embodiment, the logic circuit module 140a has an input terminal connected to the node 150, an output terminal, and a complementary output terminal. In an exemplary embodiment, when it is detected that the write detect voltage Vwd is less/greater than a threshold value, e.g., half the write voltage Vch, the logic circuit module 140a outputs low and high logic levels at the output terminal and the complementary output terminal thereof, respectively. On the other hand, in an exemplary embodiment, when it is detected that the write detect voltage Vwd is substantially equal to a threshold value, e.g., half the write voltage Vch, the logic circuit module 140a outputs high and low logic levels at the output terminal and the complementary output terminal thereof, respectively.
The transmission gate module 140b is responsive to the logic levels output by the logic circuit module 140a. In this exemplary embodiment, the transmission gate module 140b includes parallel-connected PMOS and NMOS transistors; an input terminal connected to the node 250 of the voltage-generating circuit 120; an output terminal connected to the source terminals of the first and second transistors 110a, 110b; a PMOS gate terminal connected to the output terminal of the logic circuit module 140a; and an NMOS gate terminal connected to the complementary output terminal of the logic circuit module 140a. In response to low and high logic levels applied by the logic circuit module 140a to the PMOS and NMOS gate terminals thereof, respectively, the transmission gate module 140b makes a transmission path between the input and output terminals thereof. This connects the node 250 of the voltage-generating circuit 120 to the source terminals of the first and second transistors 110a, 110b and permits application of the write voltage Vch to the source terminals of the first and second transistors 110a, 110b, whereby the write current Iwrite is generated. Conversely, in response to high and low logic levels applied by the logic circuit module 140a to the PMOS and NMOS gate terminals thereof, respectively, the transmission gate module 140b breaks the transmission path between the input and output terminals thereof. This disconnects the node 250 of the voltage-generating circuit 120 from the source terminals of the first and second transistors 110a, 110b and inhibits the application of the write voltage Vch to the source terminals of the first and second transistors 110a, 110b, thereby stopping the generation of the write current Iwrite.
FIG. 3 is a schematic diagram illustrating the device 100 connected to a memory 300 in accordance with some embodiments.
As illustrated in FIG. 3, the memory 300 is connected to the drain terminal of the first transistor 110a. In this exemplary embodiment, the memory 300 is a resistive random access memory (RRAM). In an alternative exemplary embodiment, the memory 300 is a phase change random access memory (PC-RAM), a conducting bridge random access memory (CB-RAM), or another programmable resistive memory.
FIG. 4 is a schematic diagram illustrating a RRAM cell 410 of the RRAM 300 in accordance with some embodiment.
As illustrated in FIG. 4, the RRAM 300 includes a plurality of 1T1R RRAM cells 410 (only one of the RRAM cells 410 is labeled in FIG. 4) that are arranged in a matrix of rows and columns. The RRAM cells 410 in each of the rows are connected between a corresponding one of bit lines BL0, BL1, . . . and BLN of the RRAM 300 and a corresponding one of source lines SL0, SL1, . . . and SLN of the RRAM 300. The RRAM cells 410 in each of the columns are connected to a corresponding one of word lines WL0, WL1, . . . WLN of the RRAM 300. Since the RRAM cells 410 are identical in structure and operation, only one of the RRAM cells 410 will be described herein.
The RRAM cell 410 includes a variable resistor 410a and an access transistor 410b that are connected in series between a bit line, e.g., the BL0, and a source line, e.g., the source line SL0. The access transistor 410b has a gate terminal connected to a word line, e.g., WL0. It is noted that the access transistor 410b has a negligible resistance, and thus the RRAM cell 410 has a resistance that is substantially equal to a resistance of the variable resistor 410a. 
In this exemplary embodiment, when the RRAM cell 410 is in a low resistance state, the RRAM cell 410 is set. Conversely, when the RRAM cell 410 is in a high resistance state, the RRAM cell 410 is reset.
FIG. 5 is a schematic diagram illustrating the device 100 connected to the RRAM cell 410 of the RRAM 300 in accordance with some embodiments and FIG. 6 is a flowchart of an exemplary method 600 for writing to the RRAM cell 410 of the RRAM 300 in accordance with some embodiments.
An exemplary embodiment of a method for setting a memory cell, e.g., the RRAM cell 410, of a memory, e.g., the RRAM 300, using a device, e.g., the device 100, will now be described with reference to FIG. 5 and FIG. 6.
In operation 610, the voltage-generating circuit 120 generates a write voltage Vch.
In an exemplary embodiment, the write voltage Vch is substantially equal to the sum of a threshold voltage of the first transistor 110a of the current mirror circuit 110 and a voltage across the RRAM cell 410. In such an exemplary embodiment, the threshold voltage of the second transistor 220 of the voltage-generating circuit 120 and the second input voltage Vi2 at the inverting input terminal of the operational amplifier 240 of the voltage-generating circuit 120 are respectively and substantially equal to the threshold voltage of the first transistor 110a of the current mirror circuit 110 and the voltage across the RRAM cell 410.
In operation 620, in response to low and high logic levels applied by the logic circuit module 140a of the switch circuit 140 to the PMOS and NMOS gate terminals of the transmission gate module 140b of the switch circuit 140, respectively, the transmission gate module 140b of the switch circuit 140 makes a transmission path between the input and output terminals thereof. This connects the node 250 of the voltage-generating circuit 120 to the source terminals of the first and second transistors 110a, 110b of the current mirror circuit 110 and permits application of the write voltage Vch to the RRAM cell 410.
In operation 630, a write current Iwrite is generated and flows through the first transistor 110a of the current mirror circuit 110.
In this exemplary embodiment, the bit line BL0 is connected to the drain terminal of the first transistor 110a of the current mirror circuit 110, the source line SL0 is connected to the ground, and the word line WL0 is connected to a voltage source (not shown). As a result, the write current Iwrite further flows from the bit line BL0 to the source line SL0 through the RRAM cell 410. This causes the RRAM cell 410 to transition from a high resistance state to a low resistance state. This, in turn, causes the write current Iwrite to increase.
In operation 640, a mirror current Imirror is generated and flows through the second transistor 110b of the current mirror circuit 110. The mirror current Imirror mirrors the write current Iwrite. That is, an increase in the write current Iwrite results in a corresponding increase in the mirror current Imirror.
In operation 650, the constant current source 130a of the current source circuit 130 generates a predetermined compliance current Icompliance.
In an exemplary embodiment, the first and second transistors 110a, 110b of the current mirror circuit 110 have substantially the same W/L ratio, i.e., the ratio of a width to a length of a channel of a transistor. As such, a ratio of the mirror current Imirror to a write current Iwrite is substantially equal to one. In such an exemplary embodiment, the predetermined compliance current Icompliance is about the write current Iwrite when the RRAM cell 410 is in the low resistance state, i.e., is set.
In another exemplary embodiment, the first transistor 110a of the current mirror circuit 110 has a W/L ratio M times larger than a W/L ratio of the second transistor 110b of the current mirror circuit 110. As such, a ratio of the mirror current Imirror to the write current Iwrite is 1:M. In such another exemplary embodiment, the predetermined compliance current Icompliance is about M times smaller than the write current Iwrite when the RRAM cell 410 is in the low resistance state, i.e., is set, thereby reducing a power consumption of the constant current source 130a of the current source circuit 130.
In operation 660, a write detect voltage Vwd is generated at the node 150.
As the RRAM cell 410 transitions to the low resistance state, the mirror current Imirror increases to the predetermined compliance current Icompliance. In this exemplary embodiment, as the mirror current Imirror increases to the predetermined compliance current Icompliance, the write detect voltage Vwd increases to a threshold value, e.g., half the write voltage Vch. As such, when the RRAM cell 410 transitions to the low resistance state and is thus set, the write detect voltage Vwd increases to the threshold value, e.g., half the write voltage Vch.
In operation 670, the logic circuit module 140a of the switch circuit 140 detects the write detect voltage Vwd.
In operation 680, when it is detected that the write detect voltage Vwd increases, i.e., is substantially equal, to the threshold value, e.g., half the write voltage Vch, the logic circuit module 140a of the switch circuit 140 outputs high and low logic levels at the output terminal and the complementary output terminal thereof that are applied to the PMOS and NMOS gate terminals of the transmission gate module 140b of the switch circuit 140, respectively. Otherwise, i.e., when it is detected that the write detect voltage Vwd is less than the threshold value, e.g., half the write voltage Vch, the logic circuit module 140a of the switch circuit 140 outputs low and high logic levels at the output terminal and the complementary output terminal thereof that are applied to the PMOS and NMOS gate terminals of the transmission gate module 140b of the switch circuit 140, respectively, and the flow goes back to operation 620.
In operation 690, in response to the high and low logic levels applied by the logic circuit module 140a of the switch circuit 140 to the PMOS and NMOS gate terminals thereof, respectively, the transmission gate module 140b of the switch circuit 140 breaks the transmission path between the input and output terminals thereof. This disconnects the node 250 of the voltage-generating circuit 120 from the source terminals of the first and second transistors 110a, 110b of the current mirror circuit 110 and inhibits the application of the write voltage Vch to the RRAM cell 410.
Based on the operation of the device 100, the switch circuit 140 disconnects the voltage-generating circuit 120 from the RRAM cell 410 to inhibit application of the write voltage Vch to the RRAM cell 410 when the RRAM cell 410 is set. As a result, the RRAM cell 410 is protected from over-set or stress.
FIG. 7 is a schematic diagram illustrating the device 100 connected to the RRAM cell 410 of the RRAM 300 in accordance with some embodiments and FIG. 8 is a flowchart of another exemplary method 800 for writing to the RRAM cell 410 of the RRAM 300 in accordance with some embodiments.
An exemplary embodiment of a method for resetting a memory cell, e.g., the RRAM cell 410, of a memory, e.g., the RRAM 300, using a device, e.g., the device 100, will now be described with reference to FIG. 7 and FIG. 8.
In operation 810, the voltage-generating circuit 120 generates a write voltage Vch.
In an exemplary embodiment, the write voltage Vch is substantially equal to the sum of a threshold voltage of the first transistor 110a of the current mirror circuit 110 and a voltage across the RRAM cell 410. In such an exemplary embodiment, the threshold voltage of the second transistor 220 of the voltage-generating circuit 120 and the second input voltage Vi2 at the inverting input terminal of the operational amplifier 240 of the voltage-generating circuit 120 are respectively and substantially equal to the threshold voltage of the first transistor 110a of the current mirror circuit 110 and the voltage across the RRAM cell 410.
In operation 820, in response to low and high logic levels applied by the logic circuit module 140a of the switch circuit 140 to the PMOS and NMOS gate terminals of the transmission gate module 140b of the switch circuit 140, respectively, the transmission gate module 140b of the switch circuit 140 makes a transmission path between the input and output terminals thereof. This connects the node 250 of the voltage-generating circuit 120 to the source terminals of the first and second transistors 110a, 110b of the current mirror circuit 110 and permits application of the write voltage Vch to the RRAM cell 410.
In operation 830, a write current Iwrite is generated and flows through the first transistor 110a of the current mirror circuit 110.
In this exemplary embodiment, the source line SL0 is connected to the drain terminal of the first transistor 110a of the current mirror circuit 110, the bit line BL0 is connected to the ground, and the word line WL0 is connected to a voltage source (not shown). As a result, the write current Iwrite further flows from the source line SL0 to the bit line BL0 through the RRAM cell 410. This causes the RRAM cell 410 to transition from a low resistance state to a high resistance state. This, in turn, causes the write current Iwrite to decrease.
In operation 840, a mirror current Imirror is generated and flows through the second transistor 110b of the current mirror circuit 110. The mirror current Imirror mirrors the write current Iwrite. That is, a decrease in the write current Iwrite results in a corresponding decrease in the mirror current Imirror.
In operation 850, the constant current source 130a of the current source circuit 130 generates a predetermined compliance current Icompliance.
In an exemplary embodiment, the first and second transistors 110a, 110b of the current mirror circuit 110 have substantially the same W/L ratio. As such, a ratio of the mirror current Imirror to the write current Iwrite is substantially equal to one. In such an exemplary embodiment, the predetermined compliance current Icompliance is about the write current Iwrite when the RRAM cell 410 is in the high resistance state, i.e., is reset.
In another exemplary embodiment, the first transistor 110a of the current mirror circuit 110 has a W/L ratio M times larger than a W/L ratio of the second transistor 110b of the current mirror circuit 110. As such, a ratio of the mirror current Imirror to the write current Iwrite is 1:M. In such an alternative exemplary embodiment, the predetermined compliance current Icompliance is about M times smaller than the write current Iwrite when the RRAM cell 410 is in the high resistance state, i.e., is reset, thereby reducing a power consumption of the constant current source 130a of the current source circuit 130.
In operation 860, a write detect voltage Vwd is generated at the node 150.
As the RRAM cell 410 transitions to the high resistance state, the mirror current Imirror decreases to the predetermined compliance current Icompliance. In this exemplary embodiment, as the mirror current Imirror decreases to the predetermined compliance current Icompliance, the write detect voltage Vwd decreases to a threshold value, e.g., half the write voltage Vch. As such, when the RRAM cell 410 transitions to the high resistance state and is thus reset, the write detect voltage Vwd decreases to the threshold value, e.g., half the write voltage Vch.
In operation 870, the logic circuit module 140a of the switch circuit 140 detects the write detect voltage Vwd.
In operation 880, when it is detected that the write detect voltage Vwd decreases, i.e., is substantially equal, to the threshold value, e.g., half the write voltage Vch, the logic circuit module 140a of the switch circuit 140 outputs high and low logic levels at the output terminal and the complementary output terminal thereof that are applied to the PMOS and NMOS gate terminals of the transmission gate module 140b of the switch circuit 140, respectively. Otherwise, i.e., when it is detected that the write detect voltage Vwd is greater than the threshold value, e.g., half the write voltage Vch, the logic circuit module 140a of the switch circuit 140 outputs low and high logic levels at the output terminal and the complementary output terminal thereof that are applied to the PMOS and NMOS gate terminals of the transmission gate module 140b of the switch circuit 140, respectively, and the flow goes back to operation 820.
In operation 890, in response to the high and low logic levels applied by the logic circuit module 140a of the switch circuit 140 to the PMOS and NMOS gate terminals thereof, respectively, the transmission gate module 140b of the switch circuit 140 breaks the transmission path between the input and output terminals thereof. This disconnects the node 250 of the voltage-generating circuit 120 from the source terminals of the first and second transistors 110a, 110b of the current mirror circuit 110 and inhibits the application of the write voltage Vch to the RRAM cell 410.
Based on the operation of the device 100, the switch circuit 140 disconnects the voltage-generating circuit 120 from the RRAM cell 410 to inhibit application of the write voltage Vch to the RRAM cell 410 when the RRAM cell 410 is reset. As a result, the RRAM cell 410 is protected from over-reset or stress.
FIG. 9 is a schematic diagram of another exemplary device 900 for writing to the RRAM cell 410 of the RRAM 300 in accordance with some embodiments. This exemplary embodiment differs from the previous exemplary embodiment in that the current source circuit 930 includes a first transistor 930a, a second transistor 930b, and a constant current source 930c. 
In this exemplary embodiment, each of the first and second transistors 930a, 930b is an NMOS transistor. The first transistor 930a has a drain terminal connected to the node 150, a source terminal connected to the ground, and a gate terminal. The second transistor 930b is a diode-connected transistor, and has a gate terminal and a drain terminal connected to each other, to the gate terminal of the first transistor 930a, and to the constant current source 930c, and a source terminal connected to the ground.
In an alternative embodiment, at least one of the first and second transistors 930a, 930b is a PMOS transistor.
Also, in this exemplary embodiment, the first transistor 930a has a W/L ratio N times larger than a W/L ratio of the second transistor 930b. That is, a ratio of a source current, which is generated by the constant current source 930c and which flows through the second transistor 930b, to a predetermined compliance current Icompliance, which flows through the first transistor 930a, is 1:N. As such, in this exemplary embodiment, the constant current source 930c is configured to generate a source current N times smaller than the predetermined compliance current Icompliance, thereby reducing a power consumption of the constant current source 930c of the current source circuit 930.
In an embodiment of a device for writing to a memory cell of a memory, the device includes a voltage-generating circuit, a current mirror circuit, a current source circuit, and a switch circuit. The voltage-generating circuit is configured to generate a write voltage. The current mirror circuit is configured to generate a mirror current that mirrors a write current flowing through the memory cell of the memory, and includes a first transistor and a second transistor. The first transistor has a source terminal, and a gate terminal and a drain terminal coupled to each other and to one of a bit line and a source line of the memory. The second transistor has a source terminal and a gate terminal coupled to the source terminal and the gate terminal of the first transistor, respectively, and a drain terminal coupled to a node. The current source circuit is coupled to the node and is configured to generate a compliance current. The switch circuit is coupled to the node and between the voltage-generating circuit and the source terminals of the first and second transistors, and is configured to detect a voltage at the node, to permit application of the write voltage to the memory cell of the memory when the voltage detected thereby is less or greater than a threshold value, and to inhibit the application of the write voltage to the memory cell of the memory when the voltage detected thereby increases or decreases to the threshold value.
In another embodiment of a device for writing to a memory cell of a memory, the device includes a voltage-generating circuit, a current mirror circuit, a current source circuit, and a switch circuit. The voltage-generating device is configured to generate a write voltage, and includes first transistor and a second transistor. The first transistor has a drain terminal coupled to a first node, and a gate terminal. The second transistor has a source terminal coupled to the first node, and a gate terminal and a drain terminal coupled to each other and to a second node. The current mirror circuit is coupled between one of a bit line and a source line of the memory and a third node, and is configured to generate a mirror current that mirrors a write current flowing through the memory cell of the memory. The current source circuit is coupled to the third node and is configured to generate a compliance current. The switch circuit is coupled to the third node and between the first node of the voltage-generating circuit and the current mirror circuit, and is configured to detect a voltage at the third node, to permit application of the write voltage to the memory cell of the memory when the voltage detected thereby is less or greater than a threshold value, and to inhibit the application of the write voltage to the memory cell of the memory when the voltage detected thereby increases or decreases to the threshold value.
In an embodiment of a method for writing to a memory cell of a memory, the method includes generating a write voltage; generating a write current that flows from one of a bit line and a source line of the memory; generating a mirror current that mirrors the write current; generating a compliance current; generating a write detect voltage based on the mirror current and the compliance current; detecting the write detect voltage; when it is detected that the write detect voltage is less or greater than a threshold value, permitting application of the write voltage to the memory cell of the memory; and when it is detected that the write detect voltage increases or decreases to the threshold value, inhibiting the application of the write voltage to the memory cell of the memory.
The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.

## Claims

1. A device for writing to a memory cell of a memory, the device comprising:
a voltage-generating circuit configured to generate a write voltage;
a current mirror circuit configured to generate a mirror current that mirrors a write current flowing through the memory cell of the memory, and including
a first transistor having a source terminal, and a gate terminal and a drain terminal coupled to each other and to one of a bit line and a source line of the memory, and
a second transistor having a source terminal and a gate terminal coupled to the source terminal and the gate terminal of the first transistor, respectively, and a drain terminal coupled to a node;

a current source circuit coupled to the node and configured to generate a compliance current; and
a switch circuit coupled to the node and between the voltage-generating circuit and the source terminals of the first and second transistors, and configured to detect a voltage at the node, to permit application of the write voltage to the memory cell of the memory when the voltage detected thereby is less or greater than a threshold value, and to inhibit the application of the write voltage to the memory cell of the memory when the voltage detected thereby increases or decreases to the threshold value.

2. The device of claim 1, wherein the switch circuit includes a logic circuit module configured to generate a pair of logic levels indicative of the voltage detected thereby, and a transmission gate module configured to connect and disconnect the voltage-generating circuit to and from the source terminals of the first and second transistors in response to the logic levels.

3. The device claim 2, wherein the logic levels are complementary logic levels.

4. The device of claim 1, wherein the first transistor has a W/L ratio larger than a W/L ratio of the second transistor.

5. The device of claim 1, wherein at least one of the first and second transistors of the current mirror circuit is a P-type metal-oxide-semiconductor (PMOS) transistor.

6. The device of claim 1, wherein the current source circuit includes a constant current source, and a pair of transistors that are coupled between the constant current source and the node and that have different W/L ratios.

7. The device of claim 6, wherein one of the transistors of the current source circuit is a diode-connected transistor.

8. A device for writing to a memory cell of a memory, the device comprising:
a voltage-generating circuit configured to generate a write voltage, and including
a first transistor having a drain terminal coupled to a first node, and a gate terminal, and
a second transistor having a source terminal coupled to the first node, and a gate terminal and a drain terminal coupled to each other and to a second node;

a current mirror circuit coupled between one of a bit line and a source line of the memory and a third node, and configured to generate a mirror current that mirrors a write current flowing through the memory cell of the memory;
a current source circuit coupled to the third node and configured to generate a compliance current; and
a switch circuit coupled to the third node and between the first node of the voltage-generating circuit and the current mirror circuit, and configured to detect a voltage at the third node, to permit application of the write voltage to the memory cell of the memory when the voltage detected thereby is less or greater than a threshold value, and to inhibit the application of the write voltage to the memory cell of the memory when the voltage detected thereby increases or decreases to the threshold value.

9. The device of claim 8, further comprising an operational amplifier having an input terminal coupled to the second node, and an output terminal coupled to the gate terminal of the first transistor.

10. The device of claim 8, further comprising a resistor coupled to the second node.

11. The device of claim 8, wherein at least one of the first and second transistors is a PMOS transistor.

12. The device of claim 8, wherein a ratio of the mirror current to the write current is less than one.

13. The device of claim 8, wherein the current source circuit is further configured to generate a source current, based on which the compliance current is generated, and a ratio of the source current to the compliance current is less than one.

14. The device of claim 8, wherein the switch circuit includes a logic circuit module configured to generate a pair of logic levels indicative of the voltage detected thereby, and a transmission gate module configured to connect and disconnect the first node of the voltage-generating circuit to and from the current mirror circuit in response to the logic levels.

15. The device of claim 14, wherein the logic levels are complementary logic levels.

16. A method for writing to a memory cell of a memory, the method comprising:
generating a write voltage;
generating a write current that flows from one of a bit line and a source line of the memory;
generating a mirror current that mirrors the write current;
generating a compliance current;
generating a write detect voltage based on the mirror current and the compliance current;
detecting the write detect voltage;
when it is detected that the write detect voltage is less or greater than a threshold value, permitting application of the write voltage to the memory cell of the memory; and
when it is detected that the write detect voltage increases or decreases to the threshold value, inhibiting the application of the write voltage to the memory cell of the memory.

17. The method of claim 16, further comprising generating a source current, based on which the compliance current is generated, wherein a ratio of the source current to the compliance current is less than one.

18. The method of claim 16, wherein a ratio of the mirror current to the write current is less than one.

19. The method of claim 16, further comprising generating a pair of logic levels indicative of the voltage detected thereby, wherein the application of the write voltage to the memory cell of the memory is permitted or inhibited in response to the logic levels.

20. The method of claim 19, wherein the logic levels are complementary logic levels.