SRAM device and a method of operating the same to reduce leakage current during a sleep mode

[0001] The present invention is directed, in general, to memory devices and, more specifically, to a Static Random-Access Memory (SRAM) device and a method of operation that reduces leakage current during a sleep mode.

[0002] Different types of memory are used in electronic apparatus for various purposes. Read only memory (ROM) and random-access memory (RAM) are two such types of memory commonly used within computers for different memory functions. ROM retains its stored data when power is switched off and therefore is often employed to store programs that are needed for powering-up an apparatus. ROM, however, does not accommodate writing. RAM, on the other hand, allows data to be written to or read from selected addresses associated with memory cells and, therefore, is typically used during normal operation of the apparatus.

[0003] Two common types of RAM are dynamic RAM (DRAM) and static RAM (SRAM). DRAM is typically used for the main memory of computers or other electronic apparatuses since, though it must be refreshed, it is typically inexpensive and requires less chip space than SRAM. Though more expensive and space-consumptive, SRAM does not require refresh, making it faster. These attributes make SRAM devices particularly desirable for portable equipment, such as laptop computers and personal digital assistants (PDAs).

[0004] A typical SRAM device includes a matrix of addressable memory cells arranged in columns and rows. A typical SRAM cell includes two access transistors and a flip-flop formed with two cross-coupled inverters, each inverter having a pull-down (driver) and a pull-up (load) transistor. The gates of the access transistors in each row are connected to a word line and the sources of each of the access transistors in each column are connected to either one of a bit line pair, B or B. Peripheral circuitry associated with the rows (or word lines) and peripheral circuitry associated with the columns (or bit lines) facilitate reading data from, and writing data to, the SRAM cells.

[0005] Generally, to read data from a SRAM cell, a word line driver may activate a word line according to an address decoded by a row decoder and received via a signal path that typically includes an address bus connected to the SRAM device. The access transistors turn on and connect the outputs of the flip-flop to the bit line pair sending signals representing the data in the SRAM cell to a sense amplifier coupled to the bit line pair. The sense amplifier amplifies the potential difference on the bit line pair. Data from the sense amplifier is output to the external circuitry of the associated electronic apparatus optionally through a buffer. Essentially, data is written to each SRAM cell in the opposite way.

[0006] As mentioned above, to retain the data written to the matrix of SRAM cells, or memory array, each SRAM cell must have a continuous supply of power. SRAM devices, however, are often employed within battery-powered wireless apparatus where power consumption is an important design parameter. Accordingly, wireless apparatus may be transitioned from an active mode to a standby mode of lower power consumption. As transistor size continues to diminish (e.g., 90 nm transistors), current leakage may be unacceptably high even during standby mode, requiring a transition to a still lower power consumption level, sleep mode, or data retention mode, to conserve power adequately.

[0007] Even in sleep mode, however, current leakage is a concern. The current leakage may be a combination of subthreshold leakage current, gate leakage and diode leakage current from the SRAM cell transistors. The subthreshold leakage current may be directly related to a threshold voltage of the SRAM cell transistors. Typically, an increase in the threshold voltage reduces the subthreshold leakage current. The threshold voltage may be increased by increasing a well voltage, such as an n-well voltage V_nwell, of the SRAM cell transistors. The diode leakage current may be Gate Induced Diode Leakage (GIDL) or, similarly, Gate Edge Diode Leakage (GEDL), that is a function of a voltage from a source or drain of the transistor to a back gate of the transistor. For example, the back gate, sometimes referred to as the body of the transistor, may be a substrate of the transistor for an n-channel transistor and an n-well for a p-channel transistor. In some process variations, there may be a p-well as the back gate of the n-channel transistors. In other processes, the back gate may be a conductor separated from the channel region by a dielectric. The term back gate may cover any structure in which the voltage thereof can influence the threshold voltage of a primary gate. Typically, as voltage between the source or drain and the back gate increases, the diode leakage current also increases.

[0008] Presently, to reduce current leakage during sleep mode, voltage across the SRAM cell may be reduced as limited by static noise margin (SNM) requirements and by using back gate biases as limited by diode leakage current. For example, a high operating voltage V_DD supplied to the SRAM device may be lowered to reduce voltage across the SRAM cells and the n-well voltage may be increased to raise the p-channel threshold voltage V_tp. Alternatively, a low operating voltage V_SS supplied to the SRAM device may be increased to reduce the voltage across the SRAM cell and also provide back bias across n-channel transistors to raise n-channel threshold voltage V_tn. As mentioned, the increased back bias incurs an increase in the diode leakage current that reduces or negates the benefit of reduced subthreshold current. Thus, a tradeoff often occurs between increasing the voltage of the well that increases the threshold voltage and reduces the subthreshold leakage current, the I_off current, that at the same time increases the diode leakage current.

[0009] Limiting a change in voltage to either lowering V_DD or raising V_SS simplifies the required circuitry and reduces the risk in voltage control. With continued miniaturization of future generation transistors, however, sufficiently lowering or raising V_DD or V_SS, respectively, to significantly reduce gate current may result in an increase in diode current that becomes significant relative to savings in the gate current and the subthreshold current. As devices are further scaled, heavier doping is needed to control short channel effects. This heavier doping limits the subthreshold current increase with scaling but increases the diode leakage.

[0010] Accordingly, what is needed in the art is an improved low-power SRAM device that has minimum current leakage during sleep mode. More specifically, what is needed in the art is an improved SRAM device and method of operation that reduces total current leakage, including subthreshold, gate and diode currents, for new generation transistors.

[0011] To address the above-discussed deficiencies of the prior art, the present invention provides an SRAM device and a method of operating an SRAM device. In one embodiment, the SRAM device includes (1) an SRAM array coupled to row peripheral circuitry by a word line and coupled to column peripheral circuitry by bit lines and (2) a sleep mode voltage controller configured to provide both an array high supply voltage V_ADD that is lower than a high operating voltage V_DD and an array low supply voltage V_ASS that is higher than a low operating voltage V_SS to the SRAM array during a sleep mode.

[0012] In another embodiment, the present invention provides an SRAM device including (1) an SRAM array coupled to row peripheral circuitry by a word line and coupled to column peripheral circuitry by bit lines and (2) a sleep mode voltage controller configured to provide both an array high supply voltage V_ADD and an array low supply voltage V_ASS to the SRAM array during a sleep mode and modify the array high supply voltage V_ADD and the array low supply voltage V_ASS during transition from an active mode to the sleep mode.

[0013] The present invention allows optimization of the trade-off between reducing subthreshold current leakage and increasing diode gate leakage when a threshold voltage is increased with back bias for both n-channel and p-channel transistors. The present invention may substantially minimize total leakage current including gate, diode, and subthreshold currents by both raising a low operating voltage of the SRAM array relative to V_p-well and lowering a high operating voltage of the SRAM array relative to V_n-well. As transistor miniaturization continues, the present invention may reduce concerns over diode and gate leakage by allowing heavier doping for a higher threshold voltage or improved control of short channel effects. Thus, the present advantageously allows the use of shorter gate lengths while maintaining about the same I_off. The present invention may allow optimization of the array high supply voltage V_ADD, the array low supply voltage V_ASS and a well voltage as set values for a general technology class of transistors. Additionally, the present invention may allow further optimization of the SRAM array voltages through adjustments and refinements based on specific transistor characteristics.

[0014] In another aspect, the present invention provides a method of operating an SRAM device including (1) employing in an integrated circuit an SRAM array coupled to row peripheral circuitry by a word line and coupled to column peripheral circuitry by bit lines and (2) providing both an array high supply voltage V_ADD that is lower than a high operating voltage V_DD and an array low supply voltage V_ASS that is higher than a low operating voltage V_SS to the SRAM array during a sleep mode. Through this method of operating, current leakage during the sleep mode may be reduced.

[0015] The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.

[0022] Referring initially to FIG. 1, illustrated is a circuit diagram of an embodiment of an SRAM device, generally designated 100, constructed according to the principles of the present invention. The SRAM device 100 includes an SRAM array 110, row peripheral circuitry 120, column peripheral circuitry 130 and a sleep mode voltage controller 140. Typically, the SRAM array 110 includes multiple memory cells organized in a matrix of columns and rows with corresponding multiple word lines and bit lines. For example, the SRAM array 110 may have 256 columns and 256 rows of memory cells. For ease of discussion, however, only a single memory cell 115 of the SRAM array 110 is illustrated. Accordingly, a single word line and bit line pair are illustrated and discussed along with the associated row and column peripheral circuitry 120, 130.

[0023] The SRAM device 100 may be a memory component for an associated microprocessor, digital signal processor (DSP), application-specific integrated circuit (ASIC) or larger electronic apparatus. In some embodiments, more than one SRAM device 100 may comprise the memory component. An Integrated Circuit (IC) associated with the SRAM device 100 may supply a high operating voltage V_DD and a low operating voltage V_SS, thereto. The high operating voltage V_DD and the low operating voltage V_SS may be chip supply voltages. Typically, signal paths and a data interface of the associated apparatus may be coupled to the SRAM device 100 to send address information and retrieve/send data for reading/writing the data to the memory cell 115. One skilled in the art will understand coupling of the SRAM device 100 to the associated apparatus.

[0024] The memory cell 115 may have a unique address for writing and reading the data. MOSFETs and a flip-flop may be employed by the memory cell 115 to store the data as a one or a zero. Reading and writing of the data to the memory cell 115 may be controlled by the row peripheral circuitry 120 and the column peripheral circuitry 130.

[0025] The row peripheral circuitry 120 may control activating the word line of the SRAM array 110. The row peripheral circuitry 120 may include, for example, a row pre-decoder, a row decoder, a word line driver and a keeper. The word line driver may activate the word line for reading or writing based on an address signal received via a row signal path and decoded by the row pre-decoder and the row decoder.

[0026] The column peripheral circuitry 130 may control selecting the column of the SRAM array 110 for reading and writing. The column peripheral circuitry 130 may include, for example, pre-charge circuitry, write circuitry, a column multiplexer and a sense amplifier. Additionally, the column peripheral circuitry may include address decoders for determining memory cell locations within the SRAM array 110 and control circuitry for determining between writing or reading the data. The pre-charge circuitry, the write circuitry, the column multiplexer and the sense amplifier may facilitate reading and writing data to the correct column address that has been decoded. As with the row peripheral circuitry 120, the column peripheral circuitry 130 may also include additional components that facilitate writing and reading the data that are not illustrated or discussed herein.

[0027] The sleep mode voltage controller 140 may be configured to provide both an array high supply voltage V_ADD and an array low supply voltage V_ASS to the SRAM array 110 during a sleep mode and modify the array high supply voltage V_ADD and the array low supply voltage V_ASS during transition from an active mode to the sleep mode. The sleep mode voltage controller 140 may modify the array high supply voltage V_ADD and the array low supply voltage V_ASS based on reducing current leakage of the SRAM array 110 and providing sufficient voltage across the SRAM array 110 via the array high supply voltage V_ADD and the array low supply voltage V_ASS to retain data. In some embodiments, the sleep mode voltage controller 140 provides the array high supply voltage V_ADD lower than V_n-well during the sleep mode. This may be achieved by the sleep mode voltage controller 140 raising the V_n-well to, for example, 1.8 volts. Thus, the array high supply voltage V_ADD may be at a higher value than the high operating voltage V_DD, but at a lower value than V_n-well. The sleep mode voltage controller 140 may also provide the array low supply voltage V_ASS higher than a substrate voltage during the sleep mode.

[0028] Additionally, the sleep mode voltage controller 140 may be configured to regulate the array high supply voltage V_ADD relative to the array low supply voltage V_ASS during the sleep mode. Alternatively, the sleep mode voltage controller 140 may be configured to regulate the array low supply voltage V_ASS relative to the array high supply voltage V_ADD during the sleep mode. The sleep mode voltage controller 140 may regulate the voltages to insure data retention while also minimizing leakage current. The sleep mode voltage controller 140 may regulate or modify the array high supply voltage V_ADD and the array low supply voltage V_ASS employing a low drop-out (LDO) regulator. Of course other components as discussed below with respect to the sleep mode voltage controller 140 may be employed to regulate the voltages.

[0029] In other embodiments, the sleep mode voltage controller 140 may be configured to provide both an array high supply voltage V_ADD that is lower than the high operating voltage V_DD and an array low supply voltage V_ASS that is higher than the low operating voltage V_SS to the SRAM array 110 during a sleep mode. Of course, in some embodiments an optimum bias of the SRAM array 110 may occur with the array high supply voltage V_ADD higher than the high operating voltage V_DD. The array high supply voltage V_ADD is considered “high” compared to the low operating voltage V_SS instead of as a high value of a possible range of the high operating voltage V_DD.

[0030] The sleep mode voltage controller 140 may employ a fuse(s), a ROM, a transistor, a diode, a low-drop out regulator or a combination thereof to provide the sleep mode array voltages and allow adjustments or refinements of the voltages if desired. In some embodiments, the sleep mode voltage controller 140 may provide the array high supply voltage V_ADD relative to an n-well voltage. Additionally, the sleep mode voltage controller 140 may provide the array low supply voltage V_ASS relative to a p-well or substrate voltage. For example, the sleep mode voltage controller 140 may provide the array high supply voltage V_ADD at a voltage value lower than a n-well voltage V_nwell and the array low supply voltage V_ASS higher than the substrate voltage (e.g., the low operating voltage V_SS). The relatively lower array high supply voltage V_ADD may reduce a p-channel off current while increasing the p-channel diode leakage current. Additionally, the relatively higher array low supply voltage V_ASS may reduce an n-channel off current while increasing an n-channel diode leakage current. A minimum total leakage current, however, may be obtained when both the array high supply voltage V_ADD and the array low supply voltage V_ASS are relatively lower than the well voltage and relatively higher than the substrate voltage, respectively.

[0031] The sleep mode voltage controller 140 may also provide an optimum well voltage during the sleep mode. The optimum well voltage may be determined in conjunction with the array high supply voltage V_ADD and the array low supply voltage V_ASS to provide a minimum leakage current during the sleep mode. Typically, the well voltage may be about the high operating voltage V_DD. The sleep mode voltage controller 140, therefore, may provide the array high supply voltage V_ADD and the array low supply voltage V_ASS at voltage values between the high operating voltage V_DD and the low operating voltage V_SS and provide a well voltage at about the high operating voltage V_DD. In some embodiments, the well voltage V_nwell may be about 1.2 volts. The well voltage provided by the sleep mode voltage controller 140 may also vary based on transistor parameters.

[0032] The sleep mode voltage controller 140 may also provide the array high supply voltage V_ADD and the array low supply voltage V_ASS based on transistor parameters. In some embodiments, the sleep mode voltage controller 140 may provide the array high supply voltage V_ADD and the array low supply voltage V_ASS between the substrate voltage and the well voltage V_nwell according to threshold voltages V_tn, V_tp, and diode gate leakage parameters.

[0033] In some embodiments, the sleep mode voltage controller 140 may provide the array high supply voltage V_ADD, the array low supply voltage V_ASS and the well voltage at optimum values for a set of transistor parameters. For example, the sleep mode voltage controller 140 may provide the array high supply voltage V_ADD at about 0.8 volts, the array low supply voltage V_ASS at about 0.4 volts and the n-well voltage V_nwell at about 1.2 volts for a general technology class of transistors. Thus, the SRAM array may have about 0.4 volts back bias on both the n-channel and the p-channel in addition to about 0.4 volts across the SRAM cell, as shown in FIGS. 4 and 5. As shown in FIG. 5, this embodiment provides a well voltage such that an n-channel back bias voltage, a p-channel back bias voltage, and a voltage across the SRAM cell are about (but probably not exactly) the same.

[0034] Additionally, the sleep mode voltage controller 140 may provide an adjustable array high supply voltage V_ADD and array low supply voltage V_ASS. Thus, the sleep mode voltage controller 140 may allow fine tuning of SRAM array 110 voltages for the sleep mode to obtain optimum values to reduce current leakage. The sleep mode voltage controller 140 may adjust the array high supply voltage V_ADD and the array low supply voltage V_ASS based on a process corner transistor parameter. For example, for a strong-n/weak-p corner, V_ADD and V_ASS may be made higher than for a weak-n/strong-p corner so as to apply more back bias to the n-channel transistors. Conversely, for a weak-n/strong-p corner, V_ADD and V_ASS may be made relatively lower to apply more back bias to the p-channel transistors. Similarly, a relatively higher V_ADD and relatively lower V_ASS may be applied for a weak-n/weak-p corner relative to a strong-n/strong-p corner to optimize the trade-off of diode current and subthreshold current.

[0035] Turning now to FIG. 2, illustrated is a block diagram of an embodiment of a sleep mode voltage controller, generally designated 200, constructed according to the principles of the present invention. The sleep mode voltage controller 200 includes a first, second, third and fourth switch, 210, 220, 230, 240, a first and second diode, 250, 260, and a low drop-out (LDO) regulator 270.

[0036] The first, second, and third switch 210, 220, 230, may be coupled between a supply of the high operating voltage V_DD and the array high supply voltage V_ADD. The fourth switch 240 may be coupled between the array low supply voltage V_ASS and a supply of the low operating voltage V_SS. Each one of the first, second, and third switch 210, 220, 230, may be a p-channel MOSFET and the fourth switch 240 may be n-channel MOSFET to facilitate pulling-up and pulling-down of associated voltages. The LDO regulator 270 may be connected to the array high supply voltage V_ADD, the array low supply voltage V_ASS and switch 240. The LDO regulator 270 may also be connected to a reference voltage that is not shown for clarity.

[0037] The third switch 230 may be controlled by a power-down signal PD that indicates when a SRAM array is entering the sleep mode. When in an active mode or standby mode, the power-down signal PD may be low and the third switch 230 is turned-on such that the high operating voltage V_DD is provided to the SRAM array. When entering the sleep mode, the power-down signal PD may go high and the third switch 230 is turned-off such that the high operating voltage V_DD is not provided to the SRAM array.

[0038] The first and second switches, 210, 220, may be controlled by a first and a second select signal SEL1, SEL2 that may be used to choose, adjust or refine the array high supply voltage V_ADD provided to the SRAM array by the sleep mode voltage controller 200. For example, the first select signal SEL1 may go low to turn on the first switch 210 when the power-down signal goes high. With the first switch 210 turned-on, a voltage drop across the first diode 250 may lower the high operating voltage V_DD and provide the array high supply voltage V_ADD to the SRAM array. Additionally, the LDO 270 may regulate the array low supply voltage V_ASS provided to the SRAM array relative to the array high supply voltage V_ADD. Thus, the sleep mode voltage controller 200 may allow tight control of voltage across a SRAM cell which may be critical to stability while in the sleep mode.

[0039] The second select signal SEL2 may be used to control the second switch 220 to adjust the array high supply voltage V_ADD and the array low supply voltage V_ASS based on parameters of the SRAM array transistors. Additional switches and diodes may also be included to allow additional adjustment or refinement of the array high supply voltage V_ADD and the array low supply voltage V_ASS. One skilled in the art will also understand that the sleep mode voltage controller 200 may employ other components in addition to or instead of those illustrated to adjust and refine voltages provided to the SRAM array during the sleep mode.

[0040] The sleep mode voltage controller 200 may also be configured to provide a well voltage, such as an n-well voltage V_nwell, to the SRAM array during sleep mode. As illustrated, the sleep mode voltage controller 200 provides the high operating voltage V_DD to the SRAM array for the n-well voltage during sleep mode. In other embodiments, the sleep mode voltage controller 200 may include additional switches or other controls to selectively provide a well voltage to the SRAM array based on transistor parameters. For example, n-well may be raised relative to the high operating voltage V_DD for embodiments having strong corner p-channel transistors. Optionally, the first switch 210 and the second switch 220 may be connected to a voltage other than the high operating voltage V_DD. For example, the first switch 210 and the second switch 220 may be connected to the high input/output voltage V_DDI/O. Also, as shown, first diode 250 and second diode 260 may be used to provide a voltage drop from the high operating voltage V_DD to the array high supply voltage V_ADD. Other circuit elements could be used such as transistors or resistors. Alternatively, a voltage regulator such as an LDO may be used to supply the array high supply voltage V_ADD. Similarly, various circuit elements, such as diodes, transistors, and resistors may be used to supply the array low supply voltage V_ASS.

[0041] Turning now to FIG. 3, illustrated is an embodiment of a method of operating an SRAM device, generally designated 300, carried out according to the principles of the present invention. The method begins in a step 305 with a desire to operate an SRAM device.

[0042] After beginning, an SRAM array is employed with an integrated circuit (IC) in a step 310. The SRAM array may be coupled to row peripheral circuitry and to column peripheral circuitry as part of the SRAM device. The IC may be an associated microprocessor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC) or a larger electronic apparatus. The SRAM device may be a memory component for the IC.

[0043] After employing the SRAM array, both an array high supply voltage V_ADD that is lower than a high operating voltage V_DD and an array low supply voltage V_ASS that is higher than a low operating voltage V_SS are provided to the SRAM array during a sleep mode in a step 320. In one embodiment, the array high supply voltage V_ADD and the array low supply voltage V_ASS may be provided based on transistor parameters of the SRAM array. The array high supply voltage V_ADD may be provided relative to a well voltage, such as, an n-well voltage. The array low supply voltage V_ASS may be provided relative a substrate voltage.

[0044] Additionally, the well voltage may be provided to the SRAM array during the sleep mode. In some embodiments, the well voltage may be at about the high operating voltage V_DD. The well voltage, the array high supply voltage V_ADD and the array low supply voltage V_ASS may be provided as a set of optimum values for a general technology class of transistors. A sleep mode voltage controller may be employed to provide the SRAM array voltages (the array high supply voltage V_ADD, the array low supply voltage V_ASS and the well voltage).

[0045] After providing both the array high supply voltage V_ADD and the array low supply voltage V_ASS, the array high supply voltage V_ADD and the array low supply voltage V_ASS are adjusted based on a sleep mode current in a step 330. For example, if a n-channel I_off is high, the array low supply voltage V_ASS may be raised to provide more of a back gate bias on the n-channel. If a p-channel I_off is high, the array high supply voltage V_ADD may be lowered to provide more of a back gate bias on the p-channel. If both the n-channel I_off and the p-channel I_off are high, stability of a SRAM cell may tolerate a lesser amount of voltage across the SRAM cell.

[0046] After adjusting the array high supply voltage V_ADD and the array low supply voltage V_ASS, the array high supply voltage V_ADD and the array low supply voltage V_ASS are refined based on a diode leakage current in a step 340. For example, if n-channel diode leakage current is high, the array low supply voltage V_ASS may not be raised as much. Also, if the p-channel diode leakage current is high, the array high supply voltage V_ADD may not be raised as much. Accordingly, the amount of back bias for the n-channel and the p-channel will not be as much. The sleep mode voltage controller may be employed to adjust and refine the SRAM array voltages for the sleep mode by employing fuses, transistors, diodes, a ROM, a LDO or a combination thereof. A test may be performed to determine a minimum voltage to maintain across the cell to retain data or to verify that the voltage across the cell is sufficient to retain data. The SRAM array voltages may be based on minimum cell voltage for data retention.

[0047] After refining, the method of operating the SRAM device ends in a step 350. Thus, the above method provides an embodiment to operate a SRAM device that reduces leakage current during sleep mode. While the methods disclosed herein have been described and shown with reference to particular steps performed in a particular order, it will be understood that these steps may be combined, subdivided or reordered to form an equivalent method without departing from the teachings of the present invention. Accordingly, unless specifically indicated herein, the order and/or the grouping of the steps are not limitations of the present invention. For example, the SRAM array voltages may be determined for a class of a process or design or may be done for individual instances. Furthermore, the SRAM array voltages may be determined at an initial characterization and test or may be done during operation.

[0048] Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.