System and method for level shifter

This invention relates generally to semiconductor circuits, and more particularly to a system and method for a level shifter.

As semiconductor feature sizes get smaller and smaller, the voltage level that low voltage, minimum geometry devices can withstand has decreased correspondingly. Because high voltage devices have to isolate the same voltage independent of the technology node, however, the area consumption of high-voltage devices compared to low voltage devices is steadily increasing. Thinner gate oxides and shorter channel lengths have reduced common supply voltages from the 5V and 3.3V seen a decade ago to 1.2V and below. The higher device density and faster performance of sub-micron processes have come at a cost of lower device breakdown voltages. High demand for small portable devices such as MP3 players have also increased the demand for circuits that can operate efficiently on a single battery cell.

Lower power supply voltages have posed a number of circuit design challenges and difficulties. One of these difficulties lies in the design of dense non-volatile memory. Very small feature sizes are required in order to economically fabricate and produce non-volatile memory devices having billions of memory cells on a single integrated circuit. Programming and erasing non-volatile memories, such as electrically erasable read only memory (EEPROM) and Flash memory, requires applying voltage levels higher than a typical minimum size sub-micron device can withstand. For example, write voltages of up to about 20V may be required in a process that only withstands a maximum voltage of about 2V. One solution to this problem has been to fabricate high voltage devices that can withstand the higher programming voltages and use these devices for memory array support circuitry, such as charge-pumps, level shifters and bit-line drivers, which must withstand these high voltages.

These high voltage devices come at a cost of thicker oxides and device areas that significantly exceed the device areas of minimum geometry low voltage devices. It is necessary to increase the physical dimensions of these high-voltage devices, as well as provide thicker gate oxides that will not break down or will not fail in the presence of these higher voltages. Incorporating these high voltage devices into high-density, non-volatile memories requires devoting a significant amount of silicon area to high voltage support circuitry, thereby limiting the maximum number of non-volatile memory cells that can be fabricated on a given integrated circuit.

In the field of non-volatile memories, what is needed are devices and methods of reducing the area devoted to high-voltage support circuitry.

In one embodiment, a bit-line interface is disclosed. The bit-line interface has a multiplexer having a plurality of bit-line outputs, and a write path coupled to a multiplexer signal input. The bit-line interface also has a read path coupled to the multiplexer signal input, wherein the read path and the write path share at least one component.

The foregoing has outlined, rather broadly, features of the present invention. Additional features of the invention will be described, hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an embodiment memory array and associated support circuitry;

FIG. 2 illustrates a schematic of an embodiment level shifter; and

FIG. 3 illustrates a schematic of an alternative embodiment level shifter.

Corresponding numerals and symbols in different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of embodiments of the present invention and are not necessarily drawn to scale. To more clearly illustrate certain embodiments, a letter indicating variations of the same structure, material, or process step may follow a figure number.

The making and using of embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that may be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to embodiments in a specific context, namely a level shifter for a bit-line interface for a non-volatile memory. Embodiments of this invention may also be applied to other circuits and systems that require area efficient use of high-voltage support circuitry in high-density circuit applications.

A non-volatile memory system 100 according to an embodiment of the present invention is shown in FIG. 1 . Non-volatile memory cell array 108 is interfaced to word-line decoder 102 and bit-line decoder 106 . Generally, the non-volatile memory cell array 108 is made up of individual non-volatile memory cells 114 , which are constructed in preferred embodiments as floating gate cells where charge is stored on a floating gate. In alternative embodiments, other cell architectures can be used such as a charge trapping memory device where charge is stored inside a charge trapping layer, such as a nitride layer or a nano-crystal layer.

Word-line decoder 102 controls word-lines WL 3 to W 0 coupled to the gates of a row of memory cells 114 in embodiments of the present invention. Word-line decoder 102 decodes an input address and selects the proper word-line to be asserted during a memory operation. In the illustration in FIG. 1 , only four word-lines are shown, however, in preferred embodiments of the present invention, more word-lines, for example between about 128 and about 2048 may be interfaced to the word-line decoder. In alternative embodiments, more or fewer word-lines can be interfaced to the word-line decoder.

Bit-line decoder 106 provides an interface to bit-lines BL 3 to BL 0 . Again, only four bit-lines are shown for simplicity of illustration, but more bit-lines, for example 1024 bit-lines, are used in embodiments of the present invention. Bit-lines BL 3 to BL 0 provide an interface from the drains of memory array transistors 114 to sense amplifier 112 during read operations, and to programming voltages generated by charge pump 120 during write and erase operations. Because programming voltages exceed the maximum operating voltages for minimum size low-voltage devices, circuit devices exposed to high voltages are implemented with high-voltage devices. These high voltage devices have a thicker gate oxide and longer channel widths and lengths than minimum size low-voltage devices in an embodiment process to withstand the higher electric fields caused by higher voltages and to protect devices from break-down and/or destruction.

In embodiments of the present invention, medium-voltage devices are preferably used if permitted by the voltage range, which needs to be applied to bit-lines during operation of the memory. Such medium-voltage devices can withstand gate voltages up to about 16V, and withstand drain-source voltages up to about 7V. Alternatively, high-voltage devices can be used. These high-voltage devices can withstand gate voltages up to about 16V, and withstand drain-source voltages up to about 17V in embodiments of the present invention. Low-voltage devices, on the other hand, can only withstand drain-source voltages up to about 1.5V before breaking down.

In an embodiment of the present invention, source-side injection (SSI) techniques are used to program memory array transistors 114 . One requirement of the S SI technique is that the drain of the memory cell 114 to be written is set to a high programming voltage. This high programming voltage is typically between about 6V and about 7V according to embodiments of the present invention. Alternatively, other voltages outside of this range sufficient to program memory cell array 108 can be used. In embodiments of the present invention, bit-line level shifter 110 couples a high programming voltage to bit-lines BL 3 to BL 0 , or a lower reference voltage, such as ground or a negative voltage, depending on the desired programming state.

When memory cell transistor 114 is being read, however, bit-lines BL 3 to BL 0 are precharged to the bias point of the sense amplifier and interfaced to sense amplifier 112 . Memory cells 114 programmed in one state will not conduct current and the corresponding bit-line will remain in its pre-charged state. Memory cells 114 programmed in second state will conduct current and the bit-line will be discharged. In order to determine whether the bit-lines have remained charged or have become discharged, sense amplifier 112 determines the state of the bit-lines and outputs the programmed memory state. In alternative embodiments of the present invention, other sensing schemes can be used. For example, the bias at the bit-line to be read can be regulated, while the current required to keep the bias condition is evaluated in order to determine the state of the bit-line.

When memory cell transistors 114 are being erased, an erase voltage of between 6V and 7V is applied to bit-lines BL 3 to BL 0 in a manner similar to writing memory cell array 108 described hereinabove.

In alternative embodiments of the present invention, other techniques besides SSI can be used to program memory cells 114 . For example, Fowler-Nordheim tunneling can be used to program and erase memory cells 114 . Fowler-Nordheim tunneling requires a longer programming time than SSI techniques, but requires less programming current for each cell, which makes Fowler-Nordheim tunneling suitable for Flash memory applications where large blocks of memory are programmed in parallel.

In conventional embodiments, separate circuits are typically used to interface memory cell array 108 to read, write, and erase circuitry via bit-lines BL 0 to BL 3 . For example, some conventional embodiments will wire-OR support circuitry to memory 108 . Each bit-line, or group of bit-lines, are interfaced to a separate write circuit, read circuit, and erase circuit according to conventional embodiments. Each of these conventional embodiments read, write and erase circuits, typically contain high-voltage or medium-voltage transistors. These circuits are separate in conventional embodiments because of the optimized nature of these devices. For example, the high-voltage and medium-voltage devices are optimized for a leakage current to on-current ratio required by the particular system. The many high-voltage and/or medium-voltage transistors in the support circuitry requires significant silicon area in conventional embodiments. Separate “channels” of read, write and erase functions seen in conventional embodiments require additional control signals to control the additional high-voltage and medium voltage devices. These additional control signals also require silicon area.

According to embodiments of the present invention, functions of the read, write, and erase circuitry that requires high-voltage and medium-voltage devices are combined into one bit-line level shifter circuit 110 . Combining as many possible options into one bit-line level shifter circuit 110 reduces the area devoted to support circuitry in embodiments of the present invention. Furthermore, the combination of high-voltage interface circuits into one bit-line level shifter 110 reduces the number of required select signals, which reduces the number of metallization layers and lowers dynamic power consumption. The reduction in dynamic power consumption is achieved because there are less digital signals that require toggling.

Non-volatile memory system 100 also has control logic that generates control signals for the operation of the memory, and charge pump 120 that generates the high voltage required to write and erase the memory in embodiments of the present invention.

Turning to FIG. 2 , a schematic of an embodiment bit-line level shifter 200 is shown. Level shifter 200 has a multiplexer 204 having transistors M 0 , M 1 and M 2 , a level-shifter core circuit having cross-coupled transistors M 3 and M 4 and pull-down transistors M 5 and M 6 . Select transistors M 7 and M 8 are included to select between the read mode and the write mode. Signals sel_read and sel_write preferably select a memory read mode and a memory write mode respectively, and signals Data and Dataq are used to provide data to bit-line level shifter 200 during write operations. Signals Data and Dataq are preferably generated by low-voltage standard logic devices. In embodiments of the present invention, bit-line level shifter 200 receives signals sel_read, sel_write, Data and Dataq, and asserts high-voltage and/or medium voltage levels on bit-lines BL n-1 to BL 0 . Consequently, high-voltage and/or medium-voltage devices are preferably used for all of the devices of the level shifter circuit of FIG. 2 .

In embodiments of the present invention, the drains of transistors M 0 , M 1 and M 2 in multiplexer 204 are coupled to bit-lines BL n-1 , BL n-2 and BL 0 . It should be understood that only three transistors M 0 , M 1 and M 2 are shown in FIG. 2 for simplicity of illustration, however, in embodiments of the present invention, each of the n bit-lines being served by a particular level shifter will have at least n multiplexer transistors. The number of bit-lines interface to a single level shifter is preferably between 16 and 64; however, a larger or smaller number of bit-lines may be interfaced to a particular level shifter depending on the application. The gates of transistors M 0 , M 1 and M 2 are coupled to select lines sel<n-1:0>, which are used to activate one or more of multiplexer transistors M 2 , M 1 and M 0 . The sources of multiplexer transistors M 2 , M 1 and M 0 are coupled to node 210 . In preferred embodiments of the present invention, multiplexer transistors M 2 , M 1 and M 0 are high-voltage or medium-voltage NMOS transistors. In alternative embodiments of the present invention, other device types can be used including PMOS devices, BJT's or other devices depending on the requirements of the application and the type of semiconductor process used.

Bit-line level shifter 200 also has a level shifter core circuit 202 that includes transistors M 3 , M 4 , M 5 and M 6 . Transistors M 5 and M 6 are preferably high voltage or medium voltage NMOS devices, and transistors M 3 and M 4 are preferably cross-coupled high-voltage or medium voltage PMOS devices.

In alternative embodiments of the present invention, other circuits may be used in place of level shifter core circuit 202 . For example, in some embodiments, cross-coupled transistors M 3 and M 4 may be PMOS transistors, and transistor M 5 and M 6 may be NMOS transistors, or all transistors in level shifter core 202 may be of the same transistor type. Furthermore, other known level shifter architectures known in the art may be used in alternative embodiments.

In a preferred embodiment of the present invention, the sources of cross-coupled transistors M 3 and M 4 are coupled to node V_write, which is set to a voltage of between about 6V and 7V when memory cells 114 ( FIG. 1 ) coupled to selected bit-lines are undergoing a write operation or an erase operation. The source of transistor M 6 is coupled to node Read. During write operation, node Read is coupled to a reference voltage, preferably ground, but during read operations, node Read is coupled to a sense amplifier. The gate of transistor M 6 is coupled to signal Data through select transistor M 8 , and the gate of transistor M 5 is coupled to signal Dataq, which is an inverted version of signal Data. For example, when signal Data is asserted as logic “High,” signal Dataq is asserted as logic “Low.” Likewise, when signal Data is asserted as logic “Low,” signal Dataq is asserted as logic “High.” Logic “High” and logic “Low” states are defined, in this case as according to the thresholds of standard logic devices. For example, in preferred embodiments of the present invention, logic “High” is nominally 1.5V and Logic “Low” is nominally 0V. In alternative embodiments of the present invention, these logic levels may be defined differently depending on the particular application and process.

During a write operation, sel<n-1:0>is used to select a bit-line and connect a bit-line to node 210 . Signal sel_write is set to logic “High” turning on transistor M 8 and coupling signal Data to the gate of M 6 in preferred embodiments. Signal sel_read is set to ground shutting off diode connected NMOS transistor M 7 , and node Read is grounded in preferred embodiments. If node Data is “High” and node Dataq is “Low,” transistor M 5 turns off and M 6 turns on discharging node 210 to ground. Consequently, transistor M 3 turns on pulling node 212 to voltage V_write. Because node 210 is at ground potential, any selected bit-line will be pulled to ground potential.

If node Data is “Low” and node Dataq is “High,” on the other hand, transistor M 6 turns off, and transistor M 5 turns on discharging node 212 to ground. Consequently, transistor M 4 turns on pulling node 210 to voltage V_write. Because node 210 is at V_write potential, any selected bit-line will be pulled to V_write.

In Erase operations, multiplexer select lines sel<n-1:0>are set to connect one bit-line, a plurality of bit-lines, or all bit-lines depending on the application. Node V_write is set to an erase voltage, preferably between 4V and 7V, sel_read is set to ground, sel_write is set to logic “High” coupling signal Data to the gate of M 6 . Signal Data is set to logic “Low” and signal Dataq is set to logic “High” which pulls node 210 to voltage V_write as discussed hereinabove. Consequently, voltage V_write is applied to the selected bit-line or bit-lines.

During read operations in preferred embodiments of the present invention, sel<n-1:0>is used to select one out of the n bit-lines, and V_write is set to a standby voltage. This standby voltage is preferably the low voltage supply voltage in preferred embodiments, or about 1.5V. In alternative embodiments, V_write can be grounded or even maintained at the write voltage. The purpose of setting V_write to a lower standby voltage is to save power by not operating the charge pumps. Node Read is coupled to a sense amplifier and signal sel_read is set to an auxiliary high or medium voltage in the range of 5 to 10V, which turns on transistor M 6 , thereby coupling the sense amplifier to the selected bit-line. Signal sel_write is set to logic “Low” (preferably ground), thereby shutting off transistor M 8 and effectively disconnecting signal Data from level shifter core 202 . Signal Dataq is set to logic “Low” ensuring that transistor M 5 remains shut-off. Effectively, transistors M 3 , M 4 and M 5 are shut down leaving transistor M 6 acting as a pass-gate between node 210 and the sense amplifier. In preferred embodiments of the present invention, node Read sees a higher impedance during read operations than during write or erase operations.

It can be seen that bit-line level-shifter 200 combines the high-voltage operation of write, read, and erase modes. More particularly, transistor M 6 performs two separate mode dependent functions. In the write and erase modes, transistor M 6 functions as a pull-down transistor as a part of level shifter core 202 . In the read mode, on the other hand, transistor M 6 functions as a pass transistor coupling the bit-line to a sense amplifier.

It can be seen that by using embodiments of the present invention, such as the embodiment illustrated in FIG. 2 , the number of high and medium voltage devices per bit-line can be reduced, thereby increasing the area efficiency of a memory module.

FIG. 3 illustrates an embodiment of the present invention that interfaces a bit-line level shifter 300 with sense amplifier 310 . During the read mode when signal sel_read is set to the auxiliary high or medium voltage mentioned above, NMOS pass transistor M 10 turns on, thereby coupling node Read to the input of sense amplifier 310 . Diode connected NMOS transistor M 7 provides a level shift from the auxiliary high or medium voltage to the gate of M 10 . Signal sel_write is a logic low, preferably ground, shutting off transistor M 9 . Sense amplifier 310 measures the voltage, current or charge on node Read and determines the stored state of the memory cell coupled to the bit-line. Sense amplifier 310 is designed according to conventional techniques in embodiments of the present invention, however, a variety of architectures can be used.

During write and erase modes, signal sel_read is at logic low, preferably ground, thereby shutting off pass transistor M 10 , while signal sel_write is maintained at logic “High,”thereby turning on transistor M 9 and pulling node Read to ground.

It will also be readily understood by those skilled in the art that materials and methods may be varied while remaining within the scope of the present invention. It is also appreciated that the present invention provides many applicable inventive concepts other than the specific contexts used to illustrate embodiments. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.