Abstract:
A method of biasing a circuit includes generating a control bias signal based on a difference between a leakage current of a baseline circuit and a reference signal; applying the control bias signal to a charge pump circuit to set a value of a reverse body bias voltage output from the charge pump, the control bias signal providing analog control of a digital clock of the charge pump circuit; and applying the reverse body bias voltage to a body of the baseline circuit.

Description:
This application is a continuation of U.S. patent application Ser. No. 12/825,330, filed Jun. 28, 2010, which is a continuation of U.S. patent application Ser. No. 12/380,492, filed Feb. 26, 2009, issued as U.S. Pat. No. 7,746,160, which is a continuation of U.S. patent application Ser. No. 11/478,006, filed Jun. 28, 2006, and issued as U.S. Pat. No. 7,504,876, the contents of each of which is incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to electronic circuits, and in particular to substrate bias circuits. 
     BACKGROUND 
     In state of the art process technologies such as those with line widths of 65 nanometers (nm) and 90 nanometers, there is significant sub-threshold leakage in cells even if the cell is in a steady state. Ideally in Static Random Access Memory (SRAM) memory circuits if a cell is not switching, no current should be drawn. However, as line widths get smaller, leakage becomes a problem even when the cells are not switching i.e., when the cells are in standby mode. This is a concern in battery powered and mobile applications where power conservation is of great importance. Furthermore SRAM circuits need to be able to wake from standby mode (where very little/no current is drawn) to active mode (where circuit is operating normally) very quickly. 
     A number of conventional chip leakage solutions for reducing the chip leakage power are described. In a first conventional solution the chip power supply is reduced during standby conditions and brought back to nominal voltages during active mode. Disadvantages of this first conventional solution include that the chip power supply needs to be brought back higher during active condition which means there is a startup time from standby to active. Also, this first conventional solution needs to be implemented as a die-by-die tweak because it is not a closed loop system. 
     In a second conventional solution low speed, high threshold voltage (Vt) or multiple Vt technologies are used to lower current leakage. Employing a high Vt technology slows down the entire chip, because the high threshold voltage causes all transistors to switch slowly. Solutions employing multi-Vt technologies are expensive due to the additional mask steps required to implement multi-Vt technologies during wafer manufacturing. For example, using older high line-width technologies such as 0.25 micrometer (um) or 250 nanometer (nm) technologies, a full mask set cost approximately one hundred thousand US dollars. Today a full mask set using cutting edge 65 nm technology costs approximately nine hundred thousand US dollars, with a cost of fifty thousand US dollars for each additional mask step. With such expensive technologies, using additional masks to implement multi-Vt technologies is a pricey solution. Furthermore, with multi-Vt technologies there is no feedback mechanism based on transistor leakage to control leakage current with process, voltage and temperature (PVT). 
     In a third conventional solution a constant substrate reverse bias reference voltage is set at wafer sort by doing a die by die tweak to get to the optimal substrate bias voltage for each die. This reverse bias operation uses a negative reference voltage for N-channel transistors, and uses a positive reference voltage for P-channel transistors. This die-by-die tweak costs additional test time per die, and the bias does not change automatically with voltage and temperature i.e. this is an open loop system with no feedback. 
     In a fourth conventional solution shown in  FIG. 1  a speed to voltage converter circuit  100  may be used to control power supply. The speed to voltage converter  100  comprises an input signal  110 , which may be a clock or other periodic signal, coupled to a replica delay path block  130  which introduces a propagation delay time (Tpd). The output of the replica delay path block  130  is coupled to an input of a speed to voltage converter block  140 , which generates a voltage proportional to the propagation delay time. The output of the speed to voltage converter block  140  is coupled to a first input of a comparator  150 . A reference voltage block  120  provides a bandgap reference to a second input of the comparator  150 . The output of the comparator  150  is coupled to a voltage and bias generator block  160 . The output of this bias generator block  160  is used to bias the transistors in the device. The speed (or frequency) to voltage converter can also be used to regulate speed by comparing the speed of a replica circuit with a reference voltage such as that produced by a bandgap. 
     Disadvantages of the fourth conventional solution include that since this solution changes the power supply voltage or bias based on the speed of a circuit (the delay of a delay chain), this is not a direct and accurate representation of the leakage in a device. Hence the leakage reduction is not optimal. Furthermore, delay circuits are usually slower at high temperatures making the feedback system operate as if it is a slow corner or a less leaky corner and hence the substrate bias is not applied. But the sub-threshold leakages are indeed worse at higher temperatures and hence the substrate bias should be applied. This difference in temperature dependence of delays and sub-threshold leakage make this feedback scheme unsuitable for reducing leakage power. 
     In a fifth solution, the leakage is reduced to a minimum possible level by finding a bias point based on the combination of GIDL (gate induced drain leakage), sub-threshold current and gate leakage currents. But this effectively reduces the speed of the circuit during active mode because the minimum leakage point corresponds to a slow corner which is slow in terms of speed. Hence, to get better speed during active mode, the reverse substrate bias has to be removed or reduced during active mode which takes time (typically in the high hundred of nanoseconds or low microseconds). This increases standby to active access time which is a very critical specification for memory circuits (for example, the standby to active time of memory circuits are in the single digit nanoseconds). 
     It would be desirable to have solution that reduces the chip leakage power based on a low ripple/noise feedback scheme that is derived from leakage parameters of a device and also works across process, voltage and temperature. It is also desirable that the same substrate bias be used during standby and active mode to not affect standby to active access time in high speed memories. Since memory chips are usually designed to meet speed, leakage and active power optimally at the typical corner for highest yield, the goal of the ideal feedback solution is to make fast/leaky process corners and high voltage/temperature corners look like a typical PVT corner (which has lower leakage power without sacrificing speed specifications). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (Prior Art) illustrates a conventional speed to voltage converter circuit used to control a power supply. 
         FIG. 2  illustrates an improved n-channel transistor substrate bias feedback scheme to reduce chip leakage power. 
         FIG. 3  illustrates an improved p-channel transistor substrate reverse bias feedback scheme to reduce chip leakage power. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment is described of an improved substrate bias feedback scheme to reduce chip leakage power. This invention comprises a substrate reverse bias feedback system which automatically sets the reverse bias voltage needed for the chip across process, voltage and temperature (PVT). The reverse bias is applied on the bulk/substrate of transistors to increase the threshold voltage of the transistors resulting in leakage reduction. The system uses a closed loop low ripple negative charge pump system controlled by an operational amplifier (op-amp) system to output the required substrate reverse bias voltage. The feedback system compares the leakage on a baseline device (for example a memory cell) to a target current value and automatically sets the required bias voltage on the baseline device. In one embodiment of this invention, simulations show leakage current reduction of approximately eighty five percent. 
       FIG. 2  shows an architecture of an substrate bias feedback circuit  200 . The circuit  200  comprises an operational amplifier (op-amp) system  210  having external power  201  (vcc), clock  202  (clk), reference voltage  203  (vref) and the internal regulated chip power supply  205  (vpwr) inputs. The op-amp block  210  has an output  214  (node1) which is coupled to an input of a charge pump system  230 . The charge pump system  230  further comprises power  201  (vcc) and clock  202  (clk) inputs and the node1  214  signal from block  210 . The charge pump system  230  has an output signal  234  VSUB which represents the desired substrate voltage. This signal is passed to the rest of the chip, and is also fed back to clamp system  220  and used to bias baseline transistor  240 . 
     The objective of the n-channel reverse bias system  200  is to output a reverse bias voltage  234  that will be applied to the body of the n-channel transistors in the core of the memory cell to reduce sub-threshold leakage. The system is designed such that it outputs a reverse body bias voltage at the high leakage corners i.e., fast/leaky silicon corners, high temperature and high voltage (vpwr) conditions. 
     The operational amplifier (op-amp) block  210  comprises a first operational amplifier  211  (indicated as “Opamp1”). This operational amplifier  211  along with transistor “pbias”  215  converts a reference voltage ‘Vref’  203  to a reference current equal to vref/rbias where vref is the reference voltage and rbias is the resistance of resistor ‘rbias’  216 . The output of operational amplifier  211  is a bias voltage to the gate of a p-transistor “psource”  212  to supply a reference current output through the baseline device  240 . Transistor “psource”  212 , has a source coupled to voltage vcc  201 , and a drain coupled to node2 which is the negative terminal of a second operational amplifier  213  (indicated as “Opamp2”). The reference current is compared with the leakage current through a baseline circuit  240  “baseline” (this circuit represents the N-transistors of a memory cell) to determine the amount of reverse body bias needed. The goal is to reduce the leakage through the circuit “baseline” to be less than or equal to the current set by the voltage to current converter. Though the circuit  240  named “baseline” shown in  FIG. 1  is an n-channel transistor, this can substituted with any group of n-transistors of a circuit for which the leakage is being reduced. 
     The node1 output  214  of the second operational amplifier “opamp2” provides the necessary analog control bias for the negative charge pump block  230 . By virtue of negative feedback, the second operational amplifier “opamp2” tries to maintain a voltage equal to the internal chip power supply voltage (vpwr)  205  on “node2” for measuring leakage. If the leakage current through the circuit “baseline”  240  is higher than what is set by “opamp1” 211 through the device “psource”  212 , then “node2” starts to drop below “vpwr”  205  voltage. This in turn increases the output voltage of “opamp2” 213 i.e., “node1”  214  goes high. If “node1”  214  goes high, the charge pump turns ON hard and hence starts increasing the reverse body bias on “baseline”  240 . The increase of reverse body bias in “baseline”  240  increases its threshold voltage and hence its leakage current starts to reduce. Thus the feedback system increases the reverse body bias until the leakage through instance “baseline” is less than or equal to the current set by the p-channel source “psource”  212 . In this way, the system automatically supplies the correct reverse body bias required for the n-channel transistors for various PVT. 
     Clamp system  220  comprises a resistor  221  “resprot” (in one embodiment a variable resistive divider) having a first end coupled to power  201  and a second end coupled to a substrate voltage VSUB  234 . A variable output of the resistor is coupled to a positive input of a comparator  222 . A negative input of comparator  222  is coupled to the reference voltage “vref”  203 . An output node  223  ‘enclk’ of comparator  222  is coupled to charge pump system  230 . 
     In the clamp system  220 , the comparator  222  limits the maximum reverse body bias applied to the chip for reliability reasons and to limit GIDL (Gate Induced Drain Leakage) current. During transient overshoot or if there is a process, voltage or temperature (PVT) at which the automatic reverse body bias control decides on a voltage lower than a desirable level (in one example negative 1V) and if the clamp level is set at −1V, then the comparator system disables the charge pump once the substrate reaches around −1V. The clamping level of reverse body bias voltage can be changed by options in the resistive divider “resprot”  221 . 
     The charge pump system  230  comprises a NAND gate  231  having clk  202  and clock enable enclk  223  input, an inverter  232 , an analog clock driver  233 , a charge pump cell  235  and a pump output  234 . The charge pump is ON when enclk is logic high and is OFF when enclk is logic low. The output signal ‘clkib’ of NAND gate  231  is coupled to inverter  232 . The output of inverter  232  is the clock signal ‘clki’ which is coupled to a first side analog clock driver block  233 . The output signal ‘clkib’ of NAND gate  231  is coupled to a second side of analog clock driver block  233 . The analog clock driver  233  has a first output  236  from its first side, and a second output  237  from its second side. The first output  236  and second output  237  are coupled to charge pump cell  235 . The charge pump  235  generates output  234  VSUB substrate voltage to be applied to the chip. 
     The charge pump system  230  operates by pumping the substrate to a negative voltage for n-channel reverse body bias. The analog voltage at “node1”  214  determines the amplitude of the clock from the analog clock driver  233  to the charge pump cell  235 . This analog control of the charge pump results in an ultra low ripple output (around 5 mV). In one embodiment, simulations show that the worst case leakage current (fast corners) is reduced by around 85% due to automatic body bias control. 
     The objective of the n-channel reverse bias system is to output a reverse bias voltage that will be applied to the body of the n-channel transistors in the core of the memory cell to reduce sub-threshold leakage. The system is designed such that it outputs a reverse body bias voltage at the high leakage corners i.e., fast/leaky silicon corners and high voltage/temperature conditions. 
     Advantages of the improved solution include providing an accurate low ripple substrate bias voltage to reduce chip leakage regardless of process voltage and temperature (PVT). This solution also has the advantage of setting the bias voltage based on the leakage from a real baseline device from the chip (e.g. memory cell as a baseline device). Furthermore, this system does not require any kind of die-by-die tweak for the reverse bias setting since the system determines the bias voltage across PVT automatically. This leads to reduced test time cost and better yield. A further advantage is that this system does not require expensive multilevel threshold voltage (Vt) technologies, which are hard to control. The noise injection through the substrate is negligible due to the system&#39;s ultra low ripple bias output (ripple in the order of 5 mV). The leakage reduction is very high (in the order of 85% reduction) for worst case corners. Since the target leakage current is set such that fast/leaky and high voltage/temperature corners behave like a typical PVT corner, an added advantage is that the speed of the system is not compromised since memory chips are usually optimized to meet leakage/active power and speed specifications at the typical PVT setting. This system also has the clamp system to limit the charge pump output to keep substrate bias within technology and GIDL limits. 
     Further described is a method for determining a bias voltage, comprising converting a reference voltage to a reference current, comparing the reference current to a baseline leakage current, and generating a reverse bias voltage based upon the comparison of reference current to a baseline leakage current. The method comprises converting a reference voltage to a reference current comprises providing a reference voltage to a first operational amplifier and providing the output of the operational amplifier to the gate of a bias transistor and to the gate of a source transistor. The method may further comprise where the step of converting a reference voltage to a reference current comprises providing a reference voltage to a negative input of a first operational amplifier having a positive input coupled to the drain of a bias transistor, and providing the output of the operational amplifier to the gate of a source transistor. 
     The method may further comprise comparing the reference current to a baseline leakage current comprises driving a voltage at the drain of the source transistor to a voltage power level. In the improved method generating a reverse bias voltage comprises increasing an output voltage of a second operational amplifier when a leakage current through a baseline circuit is higher than the reference current. The method may further comprise generating a reverse bias voltage comprises increasing a voltage provided to a charge pump. 
     In an alternate embodiment the improved solution can be applied not only to n-channel reverse bias system but can also be used in p-channel reverse bias system. 
       FIG. 3  shows an architecture  300  for a p-channel substrate bias feedback circuit. The circuit  300  comprises an operational amplifier (op-amp) system  310  having external power  301  (vcc), clock  302  (clk), reference voltage  303  (vref) and the internal regulated chip power supply  305  (vpwr) inputs. The op-amp block  310  has an output  314  (node1b) which is coupled to an input of a charge pump system  330 . The charge pump system  330  further comprises power  301  (vcc) and clock  302  (clk) inputs and the node1b  314  signal from block  310 . The charge pump system  330  has an output signal  334  VSUB which represents the desired substrate voltage. This signal is passed to the rest of the chip, and is also fed back to clamp system  320  and used to bias baseline transistor  340 . The operation of circuit  300  is similar to the operation of circuit  200  as described previously. 
     The n-channel reverse bias system employs a negative pump whereas the p-channel reverse bias system would employ a positive pump. The positive pump&#39;s output of higher than internal regulated supply voltage vpwr is applied to p-channel transistors&#39; substrate or body to reduce its leakage. The above discussions have concentrated on reducing chip leakage (standby power) but the same system can also be used to reduce active power. 
     For normal operation, semiconductor memory devices have highest active power for fast corners and lower active power at typical corners for a given speed specification. Since this feedback scheme is essentially trying to make fast and high voltage corners look like typical corners, the active power is also reduced because the substrate bias is kept ON during both standby and active modes. 
     Embodiments of the present invention are well suited to performing various other steps or variations of the steps recited herein, and in a sequence other than that depicted and/or described herein. In one embodiment, such a process is carried out by processors and other electrical and electronic components, e.g., executing computer readable and computer executable instructions comprising code contained in a computer usable medium. 
     For purposes of clarity, many of the details of the improved solution and the methods of designing and manufacturing the same that are widely known and are not relevant to the present invention have been omitted from the following description. 
     It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the invention. 
     Similarly, it should be appreciated that in the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.