Abstract:
A circuit and a method for adjusting the performance of an integrated circuit, the method includes: comprising: (a) measuring the performance of a first monitor circuit having at least one field effect transistor (FET) of a first set of FETs, each FET of the first set of FETs having a designed first threshold voltage; (b) measuring the performance of a second monitor circuit having at least one field effect transistor (FET) of a second set of FETs, each FET of the second set of FETs having a designed second threshold voltage, the second threshold voltage different from the first threshold voltage; and (c) applying a bias voltage to wells of the FETs of the second set of FETs based on comparing a measured performance of the first and second monitor circuits to specified performances of the first and second monitor circuits.

Description:
This Application is a continuation of U.S. patent application Ser. No. 11/424,961 filed on Jun. 19, 2006. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of integrated circuits; more specifically, it relates to methods and circuits to reduce threshold voltage tolerance and skew in integrated circuits utilizing devices having multiple different threshold voltages. 
   BACKGROUND OF THE INVENTION 
   In order to reduce power consumption and increase performance, circuits having devices with different threshold voltages have been used in different portions of the integrated circuit. Devices with low threshold voltages are faster, but have greater sub-threshold voltage leakage (consume more power) compared with devices having high threshold voltages but low sub-threshold voltage leakage. Using a mix of high threshold voltage devices on non-performance critical circuit paths and low threshold voltage devices on performance critical circuit paths can result in lower overall power consumption and higher performance than using devices having the same threshold voltages. 
   However, it is critical that the designed relationship between the different threshold voltage values of different-threshold voltage devices be maintained in the fabricated integrated circuit in order to ease timing closure during design and avoid signal propagation timing issues. Therefore, there is a need for methods and circuits for maintaining the design values and/or relationships between the different threshold voltage values of multiple threshold voltage devices. 
   SUMMARY OF THE INVENTION 
   A first aspect of the present invention is a circuit, comprising: a first set of field effect transistors (FETs) having a designed first threshold voltage and a second set of FETs having a designed second threshold voltage, the first threshold voltage different from the second threshold voltage; a first monitor circuit containing at least one FET of the first set of FETs and a second monitor circuit containing at least one FET of the second set of FETs; a compare circuit adapted to generate a compare signal based on a performance measurement of the first monitor circuit and a performance measurement of the second monitor circuit; and a control unit adapted to generate a control signal to a voltage regulator based on the compare signal, the voltage regulator adapted to supply a bias voltage to wells of FETs of the second set of FETs, the value of the bias voltage based on the control signal. 
   A second aspect of the present invention is the first aspect, wherein the compare circuit includes a first edge counter connected between the first monitor circuit and a first comparator and a second edge counter connected between a reference clock and the first comparator; and further including: an additional compare circuit including a third edge counter connected between the second monitor circuit and a second comparator and a fourth edge counter connected between the reference clock and the second comparator; and a first memory device containing a first performance specification for the first monitor circuit coupled to the first comparator and a second memory device memory containing a second performance specification for the second monitor circuit coupled to the second comparator. 
   A third aspect of the present invention is a method, comprising: providing a first set of field effect transistors (FETs) having a designed first threshold voltage and a second set of FETs having a designed second threshold voltage, the first threshold voltage different from the second threshold voltage; providing a first monitor circuit containing at least one FET of the first set of FETs and a second monitor circuit containing at least one FET of the second set of FETs; providing a compare circuit adapted to generate a compare signal based on a performance measurement of the first monitor circuit and a performance measurement of the second monitor circuit; and providing a control unit adapted to generate a control signal to a voltage regulator based on the compare signal, the voltage regulator adapted to supply a bias voltage to wells of FETs of the second set of FETs, the value of the bias voltage based on the control signal. 
   A fourth aspect of the present invention is the third aspect wherein the compare circuit includes a first edge counter connected between the first monitor circuit and a first comparator and a second edge counter connected between a reference clock and the first comparator; and further including: providing an additional compare circuit including a third edge counter connected between the second monitor circuit and a second comparator and a fourth edge counter connected between the reference clock and the second comparator; and providing a first p[device containing a first performance specification for the first monitor circuit coupled to the first comparator and a second memory device containing a second performance specification for the second monitor circuit coupled to the second comparator. 
   A fifth aspect of the present invention is a method, comprising: (a) measuring the performance of a first monitor circuit having at least one field effect transistor (FET) of a first set of FETs, each FET of the first set of FETs having a designed first threshold voltage; (b) measuring the performance of a second monitor circuit having at least one field effect transistor (FET) of a second set of FETs, each FET of the second set of FETs having a designed second threshold voltage, the second threshold voltage different from the first threshold voltage; and (c) applying a bias voltage to wells of the FETs of the second set of FETs based on comparing a measured performance of the first and second monitor circuits to specified performances of the first and second monitor circuits. 
   A sixth aspect of the present invention is the fifth aspect further including: (d) applying an additional bias voltage to wells of FETs of the first set of FETs based on the comparing the performances of the first and second monitor circuits measured in steps (a) and (b) to specified performances of the first and second monitor circuits. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is an exemplary circuit illustrating the principle for threshold voltage control according to the embodiments of the present invention; 
       FIG. 2  is a cross-sectional view illustrating the physical structure of the inverter of  FIG. 1 ; 
       FIG. 3A  is a schematic block circuit diagram of a circuit for threshold voltage control according to a first embodiment of the present invention; 
       FIG. 3B  is a schematic block circuit diagram of a circuit for threshold voltage control according to a second embodiment of the present invention 
       FIG. 4A  is a block circuit diagram of an exemplary individual compare circuit of the compare unit of  FIG. 3A ; 
       FIG. 4B  is a block circuit diagram of an exemplary individual compare circuit of the compare unit of  FIG. 3B ; 
       FIG. 5A  is a diagram illustrating the control signal generated by the control unit of  FIG. 3A  according to the first embodiment of the present invention; 
       FIG. 5B  is a diagram illustrating the control signal generated by the control unit of  FIG. 3B  according to the second embodiment of the present invention; 
       FIG. 6  is a schematic block diagram of a typical voltage regulator used to adjust well bias according to embodiments of the present invention; 
       FIG. 7A  is a diagram illustrating an exemplary floor plan of an integrated circuit chip according to the embodiments of the present invention; 
       FIG. 7B  is an exemplary cross-sectional diagram of multiple field effect transistors formed in a common well; and 
       FIG. 8  is a flowchart of the methods of controlling threshold voltages of multiple-threshold voltage devices according to the embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The performance of a field effect transistor (FET) is defined as the switching speed of the FET and the performance of a circuit is defined as the time delay of a signal propagated from an input to an output of the circuit. 
     FIG. 1  is an exemplary circuit illustrating the principle for threshold voltage control according to the embodiments of the present invention. In  FIG. 1 , an inverter  100  includes a P-channel field effect transistor (PFET) P 1  and an N-channel field effect transistor (NFET) N 1 . The source of PFET P 1  is connected to V DD , the source of NFET N 1  is connected to ground (or V SS ), the drains of PFET P 1  and NFET N 1  are connected to the output of the inverter and the gates of PFET P 1  and NFET N 1  are connected to the input of the inverter. PFET P 1  is fabricated in an N-well and NFET N 1  is fabricated in a P-well as is well known in the art. V DD  is defined as the most positive voltage level of the power supply and ground (or V SS ) is defined as the most negative voltage level of the power supply. 
     FIG. 2  is a cross-sectional view illustrating the physical structure of the inverter of  FIG. 1 . PFET P 1  includes a gate  105 , a source  110 , a drain  115 , an N-well  120  and an N-well contact  125 . NFET N 1  includes a gate  130 , a source  135 , a drain  140 , a P-well  145  and a P-well contact  150 . The electrical connections described supra in reference to  FIG. 1  are illustrated in  FIG. 2 . The crosshatched regions indicate dielectric material. 
   The threshold voltages (V t s) of PFET P 1  and NFET N 1  are determined during fabrication and are a function of the various doping levels of the source/drains, channel region and gate electrode. The V t s of PFET P 1  and NFET N 1  may be adjusted (after fabrication of the integrated circuit containing PFET P 1  and NFET N 1  is complete) by applying bias voltages V WBP  and V WBN  respectively to the N-well of PFET P 1  and the P-well of NFET N 1 . Applying a positive (reverse) bias to the N-well of PFET P 1  decreases its V t  (makes its V t  more negative) thus slowing down the PFET, while applying a negative (forward) bias to the N-well of PFET P 1  increases its V t  (makes its V t  less negative) thus speeding up the PFET. Applying a positive (forward) bias to the P-well of NFET N 1  decreases its V t  (makes its V t  less positive) thus speeding up the NFET, while applying a negative (reverse) bias to the P-well of NFET N 1  increases its V t  (makes its V t  more positive) thus slowing down the NFET. 
   When integrated circuits having multiple V t  FETs are designed, the V t s of FETs are designed to be positive or negative fractions of V DD . For example, a first FET may be designed to have a V t =V DD /3, a second FET may be designed to have a V t =V DD /4 and a third FET may be designed to have a V t =V DD /5. If V DD =1.0 volt, then the V 1 s are 0.33, 0.25 and 0.20 volts respectively. All other parameters being equal, a FET with a V t  of 0.20 volt will be faster than FETs having V t s of 0.25 volt and 0.33 volt. Further, the switching speeds of the different V t  devices are designed to be ratios of each other. However, due to process variations, there is a +/− tolerance on the actual V t  and therefore the switching speed obtained when the FETs are manufactured. This is called V t  tolerance. Further, since each FETs V t  is typically set during independent manufacturing steps, the tolerances of different FET types do not necessarily track. For example, a first V t  FET could be manufactured faster than design while a second V t  FET on the same integrated circuit chip could be manufactured slower than design. This is called V t  skew. Table I illustrates this problem. 
   
     
       
             
           
             
             
             
             
           
         
             
               TABLE I 
             
           
           
             
                 
             
             
               THRESHOLD VOLTAGES VERSUS SWITCHING SPEED 
             
           
        
         
             
                 
               Relative 
                 
                 
             
             
               Threshold 
               Switching 
               Switching Speed 
               Switching Speed 
             
             
               Voltage in volts 
               Speed 
               Tolerance 
               Range 
             
             
                 
             
             
               0.20 
               0.75 
               +/−0.15 
               0.90-0.60 
             
             
               0.25 
               1.00 
               +/−0.15 
               1.15-0.85 
             
             
               0.30 
               1.25 
               +/−0.15 
               1.40-1.10 
             
             
                 
             
           
        
       
     
   
   In table I, the middle speed FET (V t =0.25 volt) is the reference FET (Switching Speed=1.0). Thus, the actual switching speed of the fastest device (V t =0.20 volt ) can overlap the actual switching speed of the middle speed device (V t =0.25 volt ) and the actual switching speed of the middle speed device (V t =0.25 volt) can overlap the actual switching speed of the slowest device (V t =0.30 volt). This can complicate timing closure of the integrated circuit design because all combinations of V t  skew and tolerance must be accounted for, or it could upset the timing of the circuit design if the switching speed ratios are assumed to be the designed values. 
   In a first embodiment of the present invention, the performance (switching speed) of the different V t  devices is monitored. One V t  device is chosen as a reference and the well bias of the other V t  devices adjusted (which adjusts the V t  which in turn adjusts the switching speed) until the design performance ratio (design switching speed ratio) of the other V t  devices versus the reference V t  device are brought to or near to design values. PFETs and NFETs are monitored and adjusted separately. This removes V t  skew, but does not adjust for V t  tolerance. 
   In a second embodiment of the present invention the performance (switching speed) of the different V t  devices is monitored. The well bias of all different V t  devices is adjusted (which adjusts the V t  which in turn adjusts the switching speed) of the different V t  devices until the design performance specification (design switching speed specification) of all the V t  devices are brought to or near to design values. PFETs and NFETs are monitored and adjusted separately. This removes V t  skew and adjusts for V t  tolerance. 
     FIG. 3A  is a schematic block circuit diagram of a circuit for threshold voltage control according to a first embodiment of the present invention. In  FIG. 3A , a threshold voltage adjustment circuit  155 A includes an adjustable voltage regulator unit  160 A, a set of PFET monitor circuits P 1  through PN, a set of NFET monitor circuits N 1  through NM, a compare unit  165 A and a control unit  170 A. In the example of  FIG. 3A , reference monitor P 1  monitors a reference PFET having a design V t  of P 1  and reference monitor N 1  monitors a reference NFET having a design V t  of N 1 . Adjustable voltage regulator unit  160 A includes a separate N-well bias voltage regulator for each of the other different V t  PFETs having respective design V t s of P 2  through PN and a different P-well bias voltage regulator for each of the other different V t  NFETs having design V t s of N 2  through NM. Compare unit  165 A includes a separate compare circuit for each different V t  PFET (V t s P 2  through PN) and for each different V t  NFET (V t s N 2  through NM) that compares the performance of the monitored FET to the performance of the reference FET. Adjustable voltage regulator unit  160 A is coupled to a power supply (ultimately to an external power supply). 
   Each N-well and P-well bias voltage regulator generates a well bias voltage that is distributed throughout the integrated circuit chip to respective V t  devices as well as to respective monitors P 2  through PN. The output of each monitor P 2  through PN is connected to a respective compare circuit within compare unit  165 A as is the output of reference monitor P 1 . The output of each monitor N 2  through NM is connected to a respective compare circuit within compare unit  165 A as is the output of reference monitor N 1 . The output of compare unit  165 A is connected to control unit  170 A Control unit  170 A generates control signals supplied to respective well bias voltage regulators in adjustable voltage regulator unit  160 A. 
   The number (n) of PFET monitor circuits is the number of different V t  PFETs in the integrated circuit that are to be monitored and there is one monitor for each. The number (m) of NFET monitor circuits is the number of different V t  NFETs in the integrated circuit that are to be monitored and there is one monitor for each. Reference monitor P 1  monitors the performance of first (and reference) V t  PFETs having a reference design threshold voltage of P 1 . Monitor P 2  monitors the performance of second V t  PFETs having a design threshold voltage of P 2 . Monitor circuit PN monitors the performance of n th  V t  PFETs having a design threshold voltage of PN. Reference monitor N 1  monitors the performance of first (and reference) V t  NFETs having a reference design threshold voltage of N 1 . Monitor N 2  monitors the performance of second V t  NFETs having a design threshold voltage of N 2 . Monitor NM monitors the performance of m th  V t  NFETs having a design threshold voltage of NM. 
   The value of n need not be the same as the value of m. Not every different V t  NFET or different V t  PFET on the integrated circuit need be monitored or connected to threshold voltage adjustment circuit  155 A. In one example, adjustable voltage regulator unit  160 A, PFET monitor circuits P 1  through PN, NFET monitor circuits N 1  through NM, compare unit  165 A and control unit  170 A are physically located on the integrated circuit chip. In one example, PFET monitor circuits P 1  through PN, NFET monitor circuits N 1  through NM, compare unit  165 A and control unit  170 A are physically located on the integrated circuit chip while adjustable voltage regulator unit  160 A is physically located off chip. 
     FIG. 3B  is a schematic block circuit diagram of a circuit for threshold voltage control according to a second embodiment of the present invention.  FIG. 3B  is similar to  FIG. 3A  except as noted. In  FIG. 3B , a threshold voltage adjustment circuit  155 B includes an adjustable voltage regulator unit  160 B, a set of PFET monitor circuits P 1  through PN, a set of NFET monitor circuits N 1  through NM, a compare unit  165 B and a control unit  170 B. Adjustable voltage regulator unit  160 B includes a separate N-well bias voltage regulator for each of the different V t  PFETs (being monitored) and a separate P-well bias voltage regulator for each of the different V t  NFETs (being monitored). 
   Each voltage regulator unit generates a well bias voltage that is distributed throughout the integrated circuit chip to respective V t  devices as well as to respective monitors P 1  through PN and N 1  through NM. The output of each monitor P 1  through PN and N 1  through NM is connected to its respective compare circuit within compare unit  165 B where it is compared to a performance target for that V t  FET (instead of to a performance of a reference V t  FET as in the first embodiment of the present invention). The output of compare unit  165 B is connected to control unit  170 B. Control unit  170 B generates control signals supplied to respective well bias voltage regulators in adjustable voltage regulator unit  160 B. 
   In one example, adjustable voltage regulator unit  160 B, PFET monitor circuits P 1  through PN, NFET monitor circuits N 1  through NM, compare unit  165 B and control unit  170 B are physically located on the integrated circuit chip. In one example, PFET monitor circuits P 1  through PN, NFET monitor circuits N 1  through NM, compare unit  165 B and control unit  170 B are physically located on the integrated circuit chip while adjustable voltage regulator unit  160 B is physically located off chip. 
     FIG. 4A  is a block circuit diagram of an exemplary individual compare circuit of compare unit  165 A of  FIG. 3A . In  FIG. 4A , a compare circuit  180 A includes a first edge counter  185 , a second edge counter  190 , a third edge counter  186 , a comparator  195 A, a programmable memory  200 A and a voltage and temperature sense circuit  205 . The output of a monitor ring oscillator  210  (monitoring an FET having a design V t =P 2  through PN or N 2  through NM) is connected to the count input of first edge counter  185 . Ring oscillators are well known in the art and comprise an odd number of sequential inverters with the feedback of the output to the input of the first inverter. The output of first edge counter  185  (a signal indicating the latest actual edge count) is connected to first compare input of a comparator  195 A. The output of a reference monitor ring oscillator  211  (monitoring an FET having a design V t =P 1  or N 1 ) is connected to the count input of third edge counter  186 . A precise reference clock (external or internal to the integrated circuit chip) is connected to the count input of second edge counter  190 . Second edge counter  190  supplies three control signals (reset, start and stop) to first and third edge counters  185  and  186 , and a count valid signal to comparator  195 A. A third compare input of comparator  195 A is coupled to programmable memory  200 A, which contains pre-determined target performance monitor ring oscillator  210  to reference monitor oscillators  211  ratios. Voltage and temperature sense circuit  205  is connected to programmable memory  200 A and provides lookup values for selecting the appropriate target performance ratio based the temperature and the voltage supplied to the monitor ring oscillator. 
   While each compare circuit may include its own programmable memory and a voltage and temperature sense circuit, a common programmable memory may be shared between two or more of the compare circuits, a common voltage and temperature sense circuit may be shared between two or more of the compare circuits, or both a common programmable memory and a common voltage and temperature sense circuit may be shared between two or more of the compare circuits. 
   Monitor ring oscillator  210  and reference monitor ring oscillator  211  are exemplary monitor circuits. Monitor ring oscillator  210  represents any of PFET monitors P 2  through PN or NFET monitors N 2  through NM. If ring oscillator  210  represents PFET monitor P 2 , then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P 2 . If ring oscillator  210  represents PFET monitor PN, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage PN. If ring oscillator  210  represents NFET monitor N 2 , then the NFETs in the signal delay path of the ring oscillator is NFETs designed to have threshold voltage N 2 . If ring oscillator  210  represents NFET monitor NM, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage NM. If reference ring oscillator  211  represents PFET reference monitor P 1 , then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P 1 . If reference ring oscillator  211  represents NFET reference monitor circuit N 1 , then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage N 1 . The delays through monitor ring oscillator  210  and reference ring oscillator  211  varies as a function of V t  and well bias voltage (and also temperature and supply voltages as described infra). 
   In operation, second edge counter  190  issues a reset to first and third edge counters  185  and  186  to reset its count to zero. Then second edge counter  190  issues a start to first and third edge counters  185  and the first edge counter starts counting edges of the signal from monitor ring oscillator  210 , second edge counter starts counting clock edges from the reference clock and third edge counter  186  starts counting edges of the signal from reference ring oscillator  211 . When second edge counter  190  reaches a preset count value it issues a stop signal to first and second edge counters  185  and  186  and a count valid signal to comparator  195 A. Comparator  195 A compares the ratio of the actual count from first and second edge counters  185  and  186  to a value of a design ratio stored in programmable memory  200 A and then issues a digital signal to control unit  170 A (see  FIG. 3A ). The digital signal indicates the difference between the actual ratio and the design ratio. For example, if after 100 reference clock edges, the monitor actual count is 80 and the reference actual count is 20 then the ratio is 8:2. However, if the target ratio is 8:3, then the monitored FETs V t  needs to be changed to give 30 actual counts. 
   Since the design count is influenced by the actual (as opposed to the designed) V DD /V SS  voltage levels (which affects the voltages on the source, drain and gates of FETs) and PFET/NFET temperature, programmable memory  200 A includes a lookup table which comprises a two dimensional matrix of count ratios indexed in a first axis by voltage level increments and in a second axis by temperature increments. Voltage and temperature sensor circuit  205  measures the supply voltage to and temperature of the monitor circuit and passes the information to programmable memory  200 A so a temperature and voltage compensated design ratio can be passed on to comparator  195 A. There is some rounding error, dependent upon the granularity of matrix. Ratios of count values in the matrix may be obtained by simulation of the design, for example by using a software program such as SPICE (simulation program for integrated circuits emphasis). SPICE is a circuit simulator that was originally developed at the Electronics Research Laboratory of the University of California, Berkeley (1975) and now has many commercial variations. The user inputs circuit topology in spice netlist format. The simulator calculates and plots nodal voltages and currents in both time and frequency domains. 
     FIG. 4B  is a block circuit diagram of an exemplary individual compare circuit of compare unit  165 B of  FIG. 3B . In  FIG. 4B , a compare circuit  180 B includes first edge counter  185 , second edge counter  190 , a comparator  195 B, a programmable memory  200 B and voltage and temperature sense circuit  205 . The output of a monitor ring oscillator  210  (monitoring an FET having a design V t =P 1  through PN or N 1  through NM) is connected to the count input of first edge counter  185 . The output of first edge counter  185  (a signal indicating the latest actual edge count) is connected to first compare input of a comparator  195 B. A precise reference clock (external or internal to the integrated circuit chip) is connected to the count input of second edge counter  190 . Second edge counter  190  supplies three control signals (reset, start and stop) to first edge counter  185  and a count valid signal to comparator  195 B. A second compare input of comparator  195 B is coupled to programmable memory  200 B, which contains pre-determined target performance values for the monitor ring oscillator  210 . Voltage and temperature sense circuit  205  is connected to programmable memory  200 B and provides lookup values for selecting the appropriate performance target based the temperature of monitor ring oscillator  210  and the voltage supplied to the monitor ring oscillator. 
   While each compare circuit may include its own programmable memory and a voltage and temperature sense circuit, a common programmable memory may be shared between two or more of the compare circuits, a common voltage and temperature sense circuit may be shared between two or more of the compare circuits, or both a common programmable memory and a common voltage and temperature sense circuit may be shared between two or more of the compare circuits. 
   Monitor ring oscillator  210  has been described supra, however monitor ring oscillator  210  in  FIG. 4B  may represent any of PFET monitors P 1  through PN or NFET monitors N 1  through NM. If ring oscillator  210  represents PFET monitor circuit P 1 , then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P 1 . If ring oscillator  210  represents PFET monitor P 2 , then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage P 2 . If ring oscillator  210  represents PFET monitor circuit PN, then the PFETs in the signal delay path of the ring oscillator are PFETs designed to have threshold voltage PN. If ring oscillator  210  represents NFET monitor circuit N 1 , then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage N 1 . If ring oscillator  210  represents NFET monitor circuit N 2 , then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage N 2 . If ring oscillator  210  represents NFET monitor circuit NM, then the NFETs in the signal delay path of the ring oscillator are NFETs designed to have threshold voltage NM. The delay through ring oscillator  210  varies as a function of V t  and well bias voltage (and also temperature and supply voltages as described infra). 
   In operation, second edge counter  190  issues a reset to first edge counter  185  to reset its count to zero. Then second edge counter  190  issues a start to first edge counter  185  and the first edge counter starts counting edges of the signal from ring oscillator  210  and second edge counter starts counting clock edges from the reference clock. When second edge counter  190  reaches a preset count value it issues a stop signal to first edge counter  185  and a count valid signal to comparator  195 B. Comparator  195 B compares the value of the actual count from first edge counter  185  to a value of a design count stored in programmable memory  200 B and then issues a digital signal to control unit  170 B (see  FIG. 3B ). The digital signal indicates the difference between the actual edge count and the design edge count. For example, if after 100 reference clock edges, the actual count is 95 and the design count is 90 then the difference is 5 counts, indicating the monitored FETs V t  needs to changed to give 90 actual counts. 
   Since the design count is influenced by the actual (as opposed to the designed) V DD /V SS  voltage levels (which affects the voltages on the source, drain and gates of FETs) and PFET/NFET temperature, programmable memory  200 B includes a lookup table which comprises a two dimensional matrix of counts indexed in a first axis by voltage level increments and in a second axis by temperature increments. Voltage and temperature sensor circuit  205  measures the supply voltage to and temperature of the monitor circuit and passes the information to programmable memory  200 B so a temperature and voltage compensated design count can be passed on to comparator  195 B. There is some rounding error, dependent upon the granularity of matrix. Count values in the matrix may be obtained by simulation of the design, for example by using a software program such as SPICE described supra. 
   It should be understood that in  FIGS. 4A and 4B , a ring oscillator is only one example of a circuit that may be used to monitor the performance of FETs and other monitor circuits and if appropriate, other types of compare circuits may be employed to compare the actual performance/switching speed of a PFET or and NFET to the designed performance/switching speed of the PFET or NFET. 
     FIG. 5A  is a diagram illustrating the control signal generated by control unit  170 A of  FIG. 3A  according to the first embodiment of the present invention. In  FIG. 5A , control unit  170 A generates control signals VREGP 2  to VREGPN and VREGN 2  to VREGNM that are sent to respective voltage regulators in adjustable voltage regulator unit  160 A. VREGP 2  adjusts N-well bias voltage on PFETs designed to have threshold voltage P 2 . VREGPN adjusts N-well bias voltage on PFETs designed to have threshold voltage PN. VREGN 2  adjusts P-well bias voltage on NFETs designed to have threshold voltage N 2 . VREGNM adjusts P-well bias voltage on NFETs designed to have threshold voltage NM. There is no VREGP 1  or VREGN 1  signal because PFETs designed to have threshold voltage P 1  and NFETs designed to have threshold voltage N 1  are reference devices. 
   Control unit  170 A includes logic circuits that “calculate” or interface with on-chip stored software instructions to calculate an adjustment of well bias voltage. The adjustments reflect changes (if any) to be made in the actual threshold voltages of PFETs having design threshold voltages P 2  through PN so that, when changed threshold voltages P 2 ′ through PN′ are divided by the actual threshold voltage P 1 ′, the design threshold ratios discussed supra (if not the actual V t  values) are restored to within an acceptable tolerance range. For example (P 2 ′/P 1 ′)=(P 2 /P 1 ). The adjustments reflect changes (if any) to be made in the actual threshold voltages of NFETs having design threshold voltages N 2  through NM so that, when the changed threshold voltages N 2 ′ through NM′ are divided by the actual threshold voltage N 1 ′, the design threshold ratios discussed supra (if not the actual V t  values) are restored to within an acceptable tolerance range. For example (N 2 ′/N 1 ′)=(N 2 /N 1 ). 
   The signals VREGP 2  to VREGPN and VREGN 2  to VREGNM may be two-bit or multiple-bit words indicating an magnitude of increase, magnitude of decrease or no change in the well bias to be applied voltage regulators of adjustable voltage regulator unit  160 A. Control unit  170 A also generates an out of range signal, when it is not possible to adjust an individual voltage regulators output voltage any further. 
     FIG. 5B  is a diagram illustrating the control signal generated by control unit  170 B of  FIG. 3B  according to the second embodiment of the present invention. In  FIG. 5B , control unit  170 B generates control signals VREGP 1  to VREGPN and VREGN 1  to VREGNM that are sent to respective voltage regulators in adjustable voltage regulator unit  160 B. VREGP 1  adjusts N-well bias voltage on PFETs designed to have threshold voltage P 1 . VREGPN adjusts N-well bias voltage on PFETs designed to have threshold voltage PN. VREGN 1  adjusts P-well bias voltage on NFETs designed to have threshold voltage N 1 . VREGNM adjusts P-well bias voltage on NFETs designed to have threshold voltage NM. 
   Control unit  170 B includes logic circuits that “calculate” or interface with on-chip stored software instructions to calculate an adjustment of well bias voltage. The adjustments reflect changes (if any) to be made in the actual threshold voltages of PFETs having design threshold voltages P 1  through PN so that, when changed, the changed threshold voltages P 1 ′ through PN′ should result in the performance (switching speeds) of PFETs having the design threshold voltages P 1  through PN being the designed performance values or within an acceptable tolerance range of the designed performance values, although the actual threshold voltages may not be the designed values. The adjustments reflect changes (if any) to be made in the actual threshold voltages of NFETs having design threshold voltages N 1  through NM so that, when changed, the changed threshold voltages N 1 ′ through NM′ should result in the performance (switching speeds) of NFETS having design threshold voltages N 1  through NM being the designed performance values or within an acceptable tolerance range of the designed performance values, although the actual threshold voltages may not be the designed values. 
   The signals VREGP 1  to VREGPN and VREGN 1  to VREGNM may be two-bit or multiple-bit words indicating a magnitude of increase, magnitude of decrease or no change in the well bias to be applied to voltage regulators of adjustable voltage regulator unit  160 B. Control unit  170 B also generates an out of range signal, when it is not possible to adjust an individual voltage regulators output voltage any further. 
     FIG. 6  is a schematic block diagram of a typical voltage regulator used to adjust well bias according to embodiments of the present invention. In  FIG. 6 , a voltage regulator  215  includes a bandgap voltage reference  220 , a digital to analog converter  225  an operational amplifier  230 , a PFET  235  and a resistor  240 . The output of bandgap voltage reference  220  is connected to an input of digital to analog converter  225  and controls signals VREGPX or VREGNY (where X=1 to N and Y=1 to M) are connected to control pins of the digital to analog converter. The output of digital to analog converter  225  is connected to a first input of operational amplifier  230  and the output of the operational amplifier is connected to the gate of PFET  235 . The source of PFET  235  is connected to V DD  and the drain of PFET  235  is connected to ground though resistor  240 , to a second input of operational amplifier  230  and to the output of voltage regulator  215 . The output voltage regulator  215  is connected to well PX or NY corresponding to the VREG signal applied to the input of the voltage regulator. 
     FIG. 7A  is a diagram illustrating an exemplary floor plan of an integrated circuit chip according to the embodiments of the present invention. While the different V t  NFETs and PFETs may be placed anywhere on the integrated circuit chip this requires extensive wiring to distribute the well bias voltages. In one option, illustrated ion  FIG. 7A , PFETs of the same V t  are formed in common N wells  245 A 1  to  245 AN and NFETs of the same V t  are formed in common P wells  245 B 1  to  245 BM on integrated circuit chip  250 . This reduces the amount of well bias wiring from threshold voltage control circuit  155 B. Notice that if threshold voltage control circuit  155 B is replaced by threshold voltage control circuit  155 A (see  FIG. 3A ) then there are no wires to P-well  245 A 1  and N-well  245 B 1  and in fact, those transistors need not be in a common well, but may be dispersed throughout the integrated circuit chip. 
     FIG. 7B  is an exemplary cross-sectional diagram of multiple field effect transistors formed in a common well. In  FIG. 7B , a group of PFETs P 2  are formed in a common N-well  255 , which is connected to VREGP 2 . The crosshatched regions indicate dielectric material. 
     FIG. 8  is a flowchart of the methods of controlling threshold voltages of multiple-threshold voltage devices according to the embodiments of the present invention. In  FIG. 8 , after power-up, in step  300 , the performance monitor circuits monitor the performance of the various V t  NFETs and PFETs and in step  305  two options are presented. The first option is to adjust performance ratios of different V t  PFETs/INFETs relative to a reference threshold PFET/NFET performance (the reference PFET/NFET is one of the different V t  PFETs/NFETs) and the second option is to adjust the performance of all different V t  PFETs/NFETs to design values. If the first option is chosen, the method proceeds to step  310 , in which the first embodiment of the present invention described supra is employed. If the second option is chosen the method proceeds to step  315 , in which the second embodiment of the present invention described supra is employed. After step  310  or  315  is performed two additional options are presented in step  320 . The first additional option is to perform a one-time performance correction, and the second additional option is to perform continuous performance corrections. If the first additional option is chosen, the well bias voltages are adjusted by a single amount and the method terminates. Of course, the method using the first additional option may be repeated periodically. If the second additional option is chosen, the well bias voltages are adjusted incrementally by small amounts, and the monitors re-sampled and the well bias voltages adjusted incrementally in a continuous loop. If no adjustment is required (the monitor performance is already acceptable), the voltage regulators are not changed. 
   Thus, the embodiments of the present invention provide methods and circuits for maintaining the design values and/or relationships between the different threshold voltage values of multi-threshold voltage devices. 
   The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.