Patent Publication Number: US-9405311-B1

Title: Bias-temperature induced damage mitigation circuit

Description:
BACKGROUND 
     The present disclosure relates to semiconductor circuits and the testing of integrated circuits. More particularly, the present disclosure describes a circuit that operates on a semiconductor circuit during burn-in testing in order to regulate the flow of electrical current through the semiconductor circuit. In many circumstances, semiconductor circuits can experience damage or degradation during burn-in testing. Such damage can include increases in the threshold voltage for an integrated circuit or electromigration in metal lines or at metal interfaces. 
     Current mode logic (CML) circuits are typically disabled during burn-in test and thus can be particularly sensitive to the degrading effects of bias-temperature induced (BTI) damage. Regulating the flow of electrical current through CML circuits during burn-in testing can preserve their performance after burn-in testing has been completed. 
     SUMMARY 
     Certain embodiments are directed toward a circuit that regulates the flow of electrical current through an integrated circuit, the circuit containing: a sequencing circuit connected to a clock signal generator, the sequencing circuit configured to, responsive to receiving a clock signal from the clock signal generator, generate a set of sequencing signals that includes a first switching signal, a second switching signal, and a disable signal. The circuit may also contain a switching circuit connected to the sequencing circuit, the switching circuit configured to receive the first switching signal and the second switching signal. The circuit may include a current mirror connected to the switching circuit and the sequencing circuit, the current mirror configured to receive an activation signal from a current control logic circuit and to receive the disable signal. 
     Some embodiments may be directed toward a method of regulating a flow of electrical current in an integrated circuit. The method may comprise: sending, by a current control logic circuit, an activation signal; generating, by a clock signal generator, a clock signal; and generating, by a sequencing circuit, from the clock signal, a set of sequencing signals that includes a first switching signal, a second switching signal, and a disable signal. The method may also include receiving by a switching circuit, the first switching signal and the second switching signal; and receiving, by a current mirror, the activation signal and the disable signal. 
     Various embodiments are directed toward a system comprising a clock signal generator that generates a clock signal; a current control logic circuit that sends an activation signal; a sequencing circuit, connected to the clock signal generator, that generates, responsive to receiving a clock signal, a set of sequencing signals that includes a first switching signal, a second switching signal, and a disable signal. The system may also include a switching circuit, connected to the sequencing circuit, that receives the first switching signal and the second switching signal; and a current mirror connected to the switching circuit and the sequencing circuit, that receives the activation signal and the disable signal and enables the flow of electrical current through the switching circuit. 
     The above summary is not intended to describe each illustrated embodiment or every implementation of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The drawings included in the present application are incorporated into, and form part of, the specification. They illustrate embodiments of the present disclosure and, along with the description, serve to explain the principles of the disclosure. The drawings are only illustrative of certain embodiments and do not limit the disclosure. 
         FIG. 1  depicts a system in which an integrated circuit experiences a regulated flow of electrical current, the regulated flow of electrical current being driven by a current regulating circuit, according to embodiments. 
         FIG. 2  describes a method of reducing bias-temperature induced (BTI) damage in an integrated circuit, according to embodiments. 
         FIG. 3  depicts an integrated circuit in a current mode logic circuit, where the integrated circuit is divided into at least two branches, according to embodiments. 
         FIG. 4  portrays a waveform table for an integrated circuit having two branches, each branch being connected to one side of a current mode logic (CML) book, according to embodiments. 
         FIG. 5  depicts an integrated circuit in a current mode logic circuit, the integrated circuit being divided into at least four branches, according to embodiments. 
         FIG. 6  shows a waveform diagram for a current regulating circuit connected to an integrated circuit that is divided into at least four branches, according to embodiments. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION 
     The present disclosure relates to semiconductor circuits and methods for testing integrated circuits. More particularly, the present disclosure describes a circuit that operates on a semiconductor circuit during burn-in testing in order to regulate the flow of electrical current through the semiconductor circuit, as well a method of relieving high bias stress on field effect transistors in an integrated circuit during burn-in testing. While the present disclosure is not necessarily limited to such applications, various aspects of the disclosure may be appreciated through a discussion of various examples using this context. 
     Burn-in testing of integrated circuits is conducted to identity latent or benign defects that can cause chip or device failures. Burn-in testing places chips with integrated circuits in conditions of high voltage and temperature stress in order to induce failures that can appear at a later time after some period of operation. During burn-in testing, bias-temperature induced (BTI) damage can occur in field-effect transistors (FETs) when they are in an “off” state while being subjected to elevated temperatures and high bias conditions (stress). Periodically turning the FETs on can relax the stress, decreasing the likelihood that BTI damage occurs. However, CML circuits are disabled in order to reduce the likelihood of electromigration induced failure of copper interconnects, as well as reducing the risk of hot carrier induced (HCI) damage that can occur when high currents pass through FETs. The regulation of burn-in testing conditions, especially for technology nodes at or below 32 nm, may be relevant to reducing circuit damage as circuit elements scale. 
     According to embodiments of the present disclosure, it may be possible to “tune” the current-flow conditions of an integrated circuit during burn-in testing in order to reduce the relative likelihood of one of the aforementioned types of circuit damage occurring, while still placing a circuit under sufficiently stressful conditions to identify chips with latent defects that can be screened from installation in computing devices because of their elevated likelihood of failure. During burn-in testing, disabling a FET can lead to BTI damage because of the constant the high gate-to-drain V(gd) bias present during burn-in. Pulsing the gate on and off during burn-in testing can minimize BTI damage (by allowing a FET to “relax” during a portion of the burn-in time), however this may place copper interconnects at greater risk of electromigration as well as increase the risk of HCI damage as it allows currents to flow for a portion of the burn-in time. 
     A burn-in test cycle where electrical current flows part of the time, but not all of the time, can, according to embodiments, balance these various risks of damage. The amount of electrical current that can flow through branches of an integrated circuit during burn-in testing (i.e., the current flow duty cycle) to achieve a “balanced” operational regime may vary according to the circuit topology, the design rules for the circuit, the materials used to make the circuit, and the burn-in test conditions. Some embodiments may have a 50% duty cycle of current flow through the integrated circuit (i.e., through any branch at one time), while other integrated circuits may have current flow duty cycles that are much lower or much higher. The design of the sequencing circuit can, according to embodiments, significantly modify the timings of the sequencing signals that accomplish the desired current flow for a particular integrated circuit. The terms used herein to describe the sequencing circuit and the sequencing signals (switching signals and disable signal) should be interpreted broadly and not used to restrict the scope of the present disclosure to a single embodiment. 
     Current pulses through an integrated circuit can be created by making an active electrical pathway between an integrated circuit (or a branch of an integrated circuit) and a current source. The active electrical pathway may, according to embodiments, extend from a current source, through a current source FET (connected to the current source) and one or more switching FETs in a switching circuit, to the integrated circuit. There may be multiple branched pathways through a switching circuit, where each switching circuit branch can be selected by enabling or disabling one or more switching FETs with sequencing signals received from a sequencing circuit. The sequencing signals may be configured to enable coordinated operation of the switching FETs, allowing electrical current to flow in only one branch of an integrated circuit to relax the integrated circuit FETs from high V(gd) strain during a part of a clock cycle. 
     A sequencing circuit may receive a clock signal from a clock signal generator in order to generate and to sequentially modify sequencing signals that are routed to the switching circuit and to a current mirror (to the current source FET or to a disable FET). In some embodiments, a reference clock can provide a clock signal to the sequencing circuit. Some embodiments may include a ring oscillator to generate a clock signal. A sequencing circuit can modify the clock signal into various switching signals that can coordinate the operation of the current regulating circuit. Clock signal modifications can include phase changes (e.g., if the clock signal has a high clock signal voltage, some sequencing signals generated by the sequencing circuit may have low voltages) and frequency changes. While an inverter can produce a voltage state difference relative to the clock signal, a frequency divider can produce a sequencing signal that changes between high and low voltage states at different intervals than the clock signal. 
     Sequencing signals can be generated in repeatable patterns, according to embodiments of the present disclosure. Sequencing signals can be adjusted, according to the structure of the sequencing circuit and the pattern of the clock signal, to produce varying amounts of relaxation time in branches of an integrated circuit during a given regulatory cycle. A regulatory cycle consists of the smallest time in which each sequencing signal produced by the sequencing circuit proceeds from an initial state, through at least one complete sequencing of the respective sequencing signal cycle, back to the initial state. A regulatory cycle may also be described as having the same duration as the cycle of the slowest changing sequencing signal from an initial state back to the initial state, in some embodiments. 
       FIG. 1  depicts a system  100  in which an integrated circuit can undergo regulated (e.g., pulsed) current flow. The system  100  shows an integrated circuit  170  electrically connected to a voltage source  110  and to a current regulating circuit  130 . The integrated circuit may be divided into several branches (or portions, or segments), each of which can experience a brief flow of electrical current triggered by the current regulating circuit. Current flow regulation can, in various embodiments, occur by taking a clock signal output from a clock signal generator  120  (powered by the voltage source  110 ) and directing the clock signal into a current regulating circuit  130 . The clock signal generator  120  may include a reference clock for an integrated circuit, or some form of oscillator such as a ring oscillator. Various embodiments may include other forms of clock signal generators that are compatible with the different kinds of current regulating circuits that can be connected to integrated circuits during burn-in testing. 
     The current regulating circuit  130  may further include a sequencing circuit  150  that receives the clock signal from the clock signal generator  120 . The sequencing circuit  150  can generate and send a set of sequencing signals to different parts of the current regulating circuit  130  in order to activate switching circuit field effect transistors (FETs) and thus control which branch of the integrated circuit  170  can experience a flow of electrical current during a part of a clock cycle. 
     In embodiments, a clock signal generated by a clock signal generator can be a modulated voltage signal that transitions between a low clock signal voltage to a high clock signal voltage. A clock signal may include two such clock signal transitions in each clock cycle, and such clock signal transitions can be used to identify the beginning, the middle, and the end of a clock signal cycle. Clock cycles can begin with a clock signal transition, and a single clock cycle can end when the clock signal undergoes a clock signal transition (up or down in voltage) similar to the transition at the beginning of the cycle. 
     In embodiments, components that receive a clock signal can produce an output signal that responds to changes in the clock signal. Such responsive changes can occur, for example, upon a clock signal transition reaching a component to cause the component output to alter. Output signals (including switching signals and a disable signal) can trigger a response in the current regulating circuit  130  by modifying the activation status of switches (including switching FETs) in a switching circuit  160  or in the current mirror  140 . When switches change status, they can reroute the path that electrical current follows through the current regulating circuit. 
     Circuit components that can modify a clock signal to generate modified output signals include inverters and frequency dividers. Inverters can change the phase of the input signal, by converting a high voltage input signal to a low voltage output signal. Frequency dividers modify the frequency of signal transitions in the output signal relative to the input signal received by the frequency divider. In some embodiments, a sequencing circuit in a current regulating circuit may contain one or more “divide by two” frequency dividers that generate a divider output signal that changes at half the rate of the signal received by the divide by two frequency divider. According to embodiments, frequency dividers are not restricted to “divide by two” frequency dividers, but can include a wide range of frequency transformations. 
     A sequencing circuit  150  that includes inverters and frequency dividers can, in various embodiments, transform a single clock signal input into multiple output signals (a set of sequencing signals), that changes with each clock signal transition. Each sequencing signal generated by the sequencing circuit  150  can be directed to a different part of the current regulating circuit  130  in order to modulate the activity of switching circuit components in a predetermined order. 
     According to embodiments, a set of sequencing signals can follow a repeatable pattern of varying complexity, the complexity relating to the structure of the current mirror  140  and the switching circuit  160 . During burn-in testing, as the sequencing circuit produces sequencing signals, a pulse of electrical current can flow through different branches or portions of an integrated circuit. When switches such as switching FETs and a current source FET are connected in series and are activated by the sequencing signals, current can flow to relieve the high V(gd) stress on the integrated circuit FETs. In some embodiments, a disable signal (in the set of sequencing signals) may deactivate a current source FET in the current mirror  140 , preventing current from flowing through the switching circuit and any interconnected parts of the integrated circuit. 
     The current regulating circuit may, according to embodiments, be enabled by a logic circuit within or outside the current regulating circuit. A current control logic circuit  115 , whether part of or outside of the current regulating circuit  130 , can be powered by the voltage source  110  and transmit an activation signal to the current regulating circuit  130  according to circuits that are hardwired into the current control logic circuit  115 , or according to software instructions that are stored in or received by the current control logic circuit  115 . In some embodiments, the current control logic circuit  115  may connect to control circuitry between the clock signal generator  120  and the sequencing circuit  150 . Some embodiments may have a current control logic circuit that connects to routing switches (routing FETs) between an integrated circuit  170  and a switching circuit  160  that, when activated, direct electrical current to the switching circuit  160  upon receipt of the activation signal. Various embodiments of current regulating circuit  130  elements can be used to implement the regulation of current through an integrated circuit, as described herein. The particular examples described are not intended to denote limitations, but to act as examples representative of the concepts contained in the present disclosure. 
     According to embodiments, when the current control logic circuit  115  enables current flow through the current mirror  140 , and when the sequencing circuit applies a set of sequencing signals to the current mirror  140  and the switching circuit  160  (where the set of sequencing signals enables current flow through at least one switching gate in the switching circuit  160 ), electrical current can flow through one portion of the integrated circuit  170 . The plurality of sets of sequencing signals can be configured to cycle repeatedly through a pattern of sequencing signals in order to control the current mirror  140  and the switching circuit  160 . According to embodiments of the present disclosure, the current control logic circuit  115  may also regulate the clock signal output from the clock signal generator supplied to the current regulating circuit  130 . 
       FIG. 2  describes a method  200  of regulating the flow of electrical current in an integrated circuit connected to a current regulating circuit, according to embodiments of the present disclosure. The method begins in block  201 . The method may include sending an activation signal to a current regulating circuit similar to that described in  FIG. 1 , in order to initiate the period pulsed flow of electrical current in an integrated circuit, as shown in block  210 . The activation signal may be a simple voltage pulse or transition, or may be a more complicated signal, according to embodiments, that can activate the operation of a passgate FET within a current mirror in order to allow electrical current to flow through the current mirror into the integrated circuit. The activation signal can, according to embodiments, be generated by a current control logic circuit. The current control logic circuit may be a circuit element that receives operational instructions from a central processing unit or other source elsewhere in the integrated circuit, or from outside the integrated circuit. The current control logic circuit can, according to some embodiments, be a general processing logic circuit, or can be hard coded to operate when temperature limits climb above a threshold and when the integrated circuit is subjected to voltage conditions consistent with burn-in testing. 
     The method may also include generating a clock signal using a clock signal generator, as described in block  220 . The clock signal generator can be a reference clock in an integrated circuit, or can be a ring oscillator, according to embodiments. The method may also include a step wherein a sequencing circuit generates, based upon the clock signal, a set of sequencing signals that are transmitted to other parts of the current regulating circuit to operate the circuit and allow pulses of electrical current to flow through the integrated circuit, as shown in block  230 . According to embodiments, the sequencing circuit can contain inverters and frequency dividers that manipulate the clock signal (or modified signals that are generated from the clock signal) to produce modified signals that can be distributed through the current regulating circuit to operate the current regulating circuit. The sequencing circuit can be configured to automatically produce modified output signals upon receiving a clock signal input from a clock signal generator, and to produce repeating patterns of sequencing signals from the sequencing circuit components. 
     The method can further include the receiving, by a switching circuit, of some of the sequencing signals, according to block  240 . The receiving of the sequencing signals can serve to activate or deactivate switching FETs in the sequencing circuit to create active electrical pathways through the hierarchical structure of current mode logic (CML) books in a switching circuit. Some embodiments may generate a pair of switching signals that are received by the switching circuit in order to operate a pair of switching FETs in a single CML book in the switching circuit. Various embodiments may have a switching circuit that receives four (or more) switching signals, in order to coordinate the operation of stacked CML books within the switching circuit. Some switching signals received by the switching circuit may undergo signal transitions between high and low voltages with the same frequency that the clock signal undergoes signal transitions. Some switching signals, especially in multi-CML book switching circuits, can undergo signal transitions at lower frequencies than the clock signal that is received by the sequencing circuit. In some embodiments, switching signals with various signal transition frequencies can be simultaneously received by a switching circuit in order to create active electrical pathways through the switching circuit. 
     The method may also include the receiving, by a current mirror, of the activation signal and a disable signal, according to block  250 . The activation signal received by the current mirror may activate a switch such as a passgate FET in the current mirror. The passgate FET may be configured to enable a static bias voltage V(bias) or other signal to reach the current mirror bridge and activate the current source FET within the current mirror. The current mirror may also receive a disable signal in order to temporarily halt the flow of electrical current through any portion of the integrated circuit to which a current regulating circuit is connected. According to embodiments, the disable signal can be received by a disable FET in the current mirror in order to route the bias voltage signal to ground rather than activating a current source FET in the current mirror. The method ends in block  299 . 
       FIG. 3  depicts a regulated current mode logic (CML) circuit  300  containing a sequencing circuit  306  connected to a CML buffer  380  and to the disable FET  319  in the current mirror  305 . A power supply voltage source  301  connects to a first branch  390  (resistive load one, R 1 ) and a second branch  391  (resistive load two, R 2 ) of the CML buffer. The voltage source  301  may also be connected to a current source circuit  370  represented by current source  302  and source resistor  303 . The current source circuit  370  is connected to the drain of the bias FET  322  and drain of the passgate FET  330 . The source of the passgate FET  330  is connected to the gates of the bias FET  322 , drain of the disable FET  319  and gate of the CML buffer current source FET  321 . The bias FET  322  and CML Buffer current source FET  321  form the current mirror  305  that can be turned off by the disable FET  319  or the passgate FET  330 . The drain of CML Buffer current source FET  321  is connected to the sources of the switching circuit FETs  311  and  312 . The ratio of total channel width between bias FET  322  and current source FET  321  can determine the multiplication factor of the current mirror  305  and the amount of current steered to resistor loads  390  and  391 . 
     A passgate FET  330  may be positioned in the electrical circuit in order to activate or deactivate the current mirror  305 . The gate of the passgate FET  330  may be connected to the output of a current control logic  315  that sends an activation signal to the passgate FET  330  at some point before or during the process of burn-in testing in order to activate the current regulating circuit. In some embodiments, the current control logic  315  may be configured to receive an input from a sequencing circuit in order to route current through a resistive load  303  in the current source circuit  370 . In some embodiments, the current control logic circuit may be configured to receive an input from some other source in order to turn current flow through the current source circuit  370  on or off. The passgate FET  330  may also receive a deactivation signal during or after burn-in testing in order to turn off the current regulating circuit. 
     The current mirror bridge line  331  may be further connected to the drain of a disable FET  319 . The disable FET  319  may be connected, by its gate, to a sequencing circuit  306 . More particularly, a disable signal  318  generated by a sequencing circuit  306  may be directed to the gate of a disable FET  319  in order to periodically disable the flow of electrical current through the current source FET  321  during burn-in testing. 
     The sequencing circuit  306  in a current regulating circuit  300  can have a number of circuit elements that drive the operation of the current mirror  305  and the switching circuit  310  to create active electrical pathways through portions of the integrated circuit being tested. A clock signal generator  339  can be configured to produce a clock signal output that is received by the sequencing circuit  306 . The sequencing circuit  306  can include several circuit elements that modify the clock signal to create sequencing signals that coordinate the operation of the current regulating circuit  300 . Sequencing signals can include a disable signal  318  and a plurality of switching signals that activate switching FET gates in the switching circuit  310 . The outputs of the various circuit elements in the sequencing circuit  306  can be connected to the various FET gates in the current regulating circuit  300  to convey the sequencing signals created from the clock signal input. 
     The precise configuration of sequencing circuits may be simple or quite complex, according to embodiments. The present disclosure is intended to describe the characteristics of a sequencing circuit without limiting the possible embodiments that may actually create sequencing signals that drive the coordinated operation of a current regulating circuit as described herein. In some embodiments, a sequencing circuit can have two inverters  341  and  342  connected in sequence between the clock signal generator  339  and a divide by two frequency divider  343 . Each inverter can be configured to receive an input and to provide a modified output in an opposite phase (e.g., a high voltage input to an inverter can produce a low voltage output) from the inverter&#39;s input. In embodiments, the output of first inverter  341  (first switching signal  311 , V 1 ) can be directed to the gate of first switching FET  316  in the switching circuit  310 . The output of second inverter  342  (second switching signal  312 , V 2 ) can be directed to the gate of a second switching FET  317  in the switching circuit  310 . A pair of switching FETs that have interconnected sources, such as first switching FET  316  and second switching FET  317 , form a current mode logic book (CML book)  395  in the switching circuit  310 . A switching circuit with a single CML book can be called a one-level switching circuit. In some embodiments, CML books can be connected to each other where the interconnected sources of one CML book further connect to the source of one side of another CML book, forming a branched circuit structure that connects to portions of an integrated circuit. A switching circuit with two CML books connected in series can be called a two-level switching circuit. For an example of a two-level switching circuit with this branched structure, see CML books  591  and  593  as shown in  FIG. 5 . 
     In some embodiments, the two sides of a CML book are operated individually because the individual switching signals that activate the two sides of the CML book are out of phase with each other. For example, a clock signal and a second switching signal  312  may be in phase because the second switching signal has been generated by passing the clock signal through two inverters in the sequencing circuit  306 . The first switching signal  311  can generally be described as out of phase with the clock signal because it is generated by passing the clock signal through a single inverter in the sequencing circuit  306 . 
     Some embodiments may use the clock signal instead of the output of a second inverter in order to drive a second switching FET  317  because the clock signal and the output of a second inverter can have high and low values at the same time (the clock signal and second inverter output are in phase with each other). Each of the first and second switching signals can, according to embodiments, activate one switching FET in a CML book to create an active electrical pathway through one branch of the CML book. In some embodiments, such as those containing two-level switching circuits similar to that shown in  FIG. 5 , CML books may be interconnected such that an active electrical pathway is created only when one gate in each of two CML books connected in series are switched on or enabled. For example, when the gates on switching FETs  526  and  529  are simultaneously activated by high voltage switching signals, electrical current can flow through the switching circuit from the current source FET to the connected branch of the integrated circuit (provided that the current source FET is activated and the disable signal has a low voltage). 
     According to embodiments, a divide by two frequency divider  343  can receive a signal having a particular signal transition frequency and produce a modified output signal with half the signal transition frequency of the received signal. Some embodiments of current regulating circuits may have frequency dividers that perform frequency reductions at different ratios compared to the input signal (e.g., divide by three, divide by four, divide by eight, etc.). 
     As described herein, the disable signal  318  generated by the frequency divider  343  may be transmitted to a disable FET  319  in a current mirror  305 . The disable signal  318  can have a different transition frequency than the switching signals generated prior to the frequency divider  343  in the switching circuit  306 . Thus, the first switching signal  311  and the second switching signal  312  can each activate, in turn, a switching FET in the CML book  395  while the disable signal  318  has a low voltage. Upon the disable signal  318  transitioning to a high voltage, the current source FET  321  is deactivated and the first and second switching signals can again cycle through the respective high voltage parts of their signals even though the current cannot flow through the current mirror  305 , switching circuit  310 , and the integrated circuit. The design of the sequencing circuit  306  can be such that this cycle of FET activation and deactivation continues in a predictable pattern for as long as the current control logic  315  applies the activation signal (or does not send the deactivation signal) to the passgate FET  330 . 
       FIG. 4  portrays a waveform table  400  for a current regulating circuit connected to an integrated circuit with two branches (e.g., with a single-level switching circuit), according to embodiments. The voltages, currents, and clock signal described in the waveform table  400  generally correspond to voltages, currents, and clock signals that can be observed in embodiments of the present disclosure that correlate with integrated circuits and current regulating circuits similar to those described in  FIG. 3 . 
     The first switching signal  491 , second switching signal  492 , and disable signal  494  are sequencing signals that control the performance of the current regulating circuit. A clock signal  497  is received by a sequencing circuit from a clock signal generator. A bias voltage (V(bias))  493  can be measured at the gate of a current source FET similar to current source FET  321  in  FIG. 3 . The flow of electrical current through a first branch of an integrated circuit ( 495  I 1 ) and a second branch of the integrated circuit ( 496  I 2 ) can be measured between first branch  390  and the first switching FET  316 , and the second branch  391  and the second switching FET  317 , respectively. A first regulatory cycle  410  includes a first clock cycle  405  and a second clock cycle  406  and has four clock cycle transitions  440 ,  441 ,  442 , and  443 . A second regulatory cycle  420  has four clock cycle transitions  444 ,  445 ,  446 , and  447  and the patterns of the sequencing signals, V(bias), and currents match the patterns observed in the first regulatory cycle  410 . 
     At the bottom of the waveform table  400 , the voltage of a clock signal  497  is portrayed over a plurality of clock cycles as it undergoes multiple clock signal transitions  440  through  447 . The clock signal  497  alternates between a low clock signal voltage and a high clock signal voltage two times during a clock cycle: once in the middle, and once at the end. According to embodiments of current regulating circuits such as those portrayed in  FIG. 3  (with one-level switching circuits), when clock signal  497  has a low clock signal voltage, during the first half of a clock signal such as between clock signal transitions  440  and  441 , or clock signal transitions  442  and  443 , the first switching signal  491  (V 1 ) has a high voltage because it is created by routing the clock signal through a single signal inverter such as inverter  341  in  FIG. 3 . During the second half of a clock cycle such as between clock signal transitions  441  and  442 , or clock signal transitions  443  and  444 , the clock signal has a high clock signal voltage, causing the inverter-modified first switching signal  491  to change to have a low voltage. The first switching signal  491  and the second switching signal  492  change between high and low voltage states with each transition of the clock signal between high and low clock signal voltage states. 
     A second switching signal  492  (V 2 ) can be generated in a sequencing circuit by taking the first switching signal  491  and passing it through a second inverter such as inverter  342  in  FIG. 3 . According to embodiments, when clock signal  497  has a low voltage (and the first switching signal  491  has a high voltage, as is the case after clock signal transitions  440 ,  442 ,  444 , and  446 ), the second switching signal  492  will be low. Further, when clock signal  497  has a high voltage as in the second half of a clock cycle (as after clock signal transitions  441 ,  443 ,  445 ), the second switching signal  492  will, like the clock signal, have a high voltage. 
     When a current regulating circuit is activated, such as by receiving an activation signal from a current control logic circuit  315  at passgate FET  330  in  FIG. 3 , a bias voltage (V(bias))  493  is applied to both the gate of the bias FET  322  and the gate of the current source FET  321  in the current mirror of the current regulating circuit. Accordingly, the bias voltage  493  will be high until the disable FET (see disable FET  319  in  FIG. 3 ) is activated. A disable FET can be activated when the disable signal  494  has a high voltage, causing current to flow through the disable FET and lowering the voltage on the current source FET  321  gate. While the first switching signal  491  and the second switching signal  492  change states (e.g., from high to low, or from low to high) twice during a single clock cycle, the bias voltage V(bias)  493  and the disable signal  494  change state once during a clock cycle. 
     In embodiments such as that shown in  FIG. 3 , a regulatory cycle corresponds to the passage of the disable signal  494  through a complete signal pattern and back to the initial state that existed at the start of the regulatory cycle. The periodic interruption of burn-in testing by flowing electrical current through branches of the integrated circuit can mitigate or eliminate the bias-temperature induced damage that occurs in extended burn-in test scenarios, and can maintain FETs in operable condition. In some embodiments, the duty cycle of current flow through any branch of the integrated circuit can be 50% or less, depending on the configuration of the sequencing circuit used to generate the disable signal that regulates the disable FET. 
     The waveform table  400  indicates that electrical current can flow through the branches of an integrated circuit attached to a current regulating circuit when the bias voltage V(bias)  493  at the current source FET gate is high, and when one of the signal voltages is also high. Thus, between clock signal transitions  440  and  441  (and between  444  and  445 ), electrical current can flow through a first branch of an integrated circuit, while between clock signal transitions  441  and  442  (and between  445  and  446 ) electrical current can flow through a second branch of an integrated circuit. Between clock signal transitions  442  and  444  (and between  446  and  448 ), current does not flow through the integrated circuit because the disable signal  494  has a high voltage and V(bias)  493  is low. 
       FIG. 5  shows a circuit  500  wherein a current mode logic (CML) circuit  570  with an integrated circuit  510  having four branches: a first branch  511 , a second branch  512 , a third branch  513 , and a fourth branch  514 , is connected to a sequencing circuit  550  that can coordinate pulses of electrical current through the branches. Integrated circuit  510  is connected to a switching circuit  520 , each branch in the integrated circuit being connected to one side of a current mode logic (CML) book. The circuit further includes a clock signal generator  540 , a sequencing circuit  550 , and a current mirror  530  having a bias FET (not shown), a current source FET  531  and a disable FET  532 . The clock signal generator  540  generates a clock signal that is transmitted to the sequencing circuit  550 . 
     The sequencing circuit  550 , upon receiving the clock signal, generates, based upon the clock signal, a set of sequencing signals that control the operation of field effect transistors in the switching circuit  520  and in the current mirror  530 . The sequencing circuit  550  is configured to generate a repeating pattern of sequencing signals, where each sequencing signal repeats at least one time during the course of a regulatory cycle as the current regulating circuit operates during burn-in testing. In current regulating circuits with two-level switching circuits, an active electrical pathway can be formed by activating, in addition to the current source FET  531 , two switching FETs connected in series between a branch of the integrated circuit  510  and the current source FET  531 . 
     The switching circuit  520  has a first level  521  and a second level  522  of switching FETs, arranged into three current mode logic (CML) books. The first level includes a first CML book  591  and a second CML book  592 . The second level has a third CML book  593 . Each current mode logic book includes a pair of switching FETs, electrically interconnected at their sources to form a branched circuit structure. Upon receiving switching signals at the gates of the switching FETs in a CML book, one of the two switching FETs in the CML book can be activated and while the other remains in a resting state. The first CML book  591  includes a first switching FET  524  and a second switching FET  525  interconnected at their sources. The second CML book  592  includes a third switching FET  526  and a fourth switching FET  527 , also interconnected at their sources. In the second level  522 , a third CML book  593  is made of a fifth switching FET  528  and a sixth switching FET  529 , interconnected at their sources. 
     A hierarchical switching structure can be created by joining the interconnected sources of the first CML book  591  with the drain of the fifth switching FET  528 , the interconnected sources of the second CML book  592  with the drain of the sixth switching FET  529 , and the interconnected sources of the third CML book  593  with the drain of the current source FET  531 . Branches, or portions, of the integrated circuit  510  connect to the various switching FETs as follows: the first branch  511  connects to the drain of the first switching FET  524 ; the second branch  512  of the integrated circuit connects to the drain of the second switching FET  525 ; the third branch  513  of the integrated circuit connects to the drain of the third switching FET  526 ; and the fourth branch  514  of the integrated circuit connects to the drain of the fourth switching FET  527 . 
     The current mirror  530  includes a current source FET  531  and a disable FET  532 . The drain of the disable FET  532  is electrically connected to the bias FET (not shown in  FIG. 5 ) and to the gate of the current source FET  531 . When a current regulating circuit is activated, a bias voltage (V(bias)) can be applied to the gate of the current source FET  531 , activating the current source FET and enabling current flow through some branch of the integrated circuit  510  selected by the switching FETs activated by the present switching signals from the sequencing circuit. When a disable signal  533  (with a high voltage) is received from the sequencing circuit, however, the current source FET  531  is deactivated and current flow through the switching FETs ceases. 
     The clock signal generator  540 , electrically connected to the sequencing circuit  550 , provides a signal that can be transformed into sequencing signals by the sequencing circuit. A first inverter  551  input may receive the clock signal  540  and invert the clock signal to create a first modified signal, the first switching signal V 1 . The first inverter  551  output can be connected to the gates of the first switching FET  524  and the third switching FET  526  in order to provide the first switching signal to the gates of the first and third switching FETs, respectively. The output of the first inverter  551  may also be connected to second inverter  552  input and the signal converted again, to produce a second modified signal, the second switching signal V 2 . The output of the second inverter may be electrically connected to the gates of the second switching FET  525  and the fourth switching FET  527  in order to activate them during part of a clock cycle. 
     The second inverter  552  output may be connected to the input of a first divide by two frequency divider  553  such that the first frequency divider  553  may receive the second modified output (the second switching signal V 2 ) and generate a third modified output, a third switching signal V 3 . Because the third switching signal V 3  has passed through divide by two frequency divider  553 , it transitions between high voltage and low voltage states at half the signal transition frequency of the clock signal, the first switching signal, and the second switching signal. The third switching signal V 3  can be directed to the gate of the fifth switching FET  528  (in the third CML book,  593 ) in order to activate the fifth switching FET  528  according to the voltage of the third switching signal V 3 . The first divide by two frequency divider  553  output can be connected to the input of a third inverter  554  in order to provide the third inverter  554  with a frequency divided input signal that acts as the third switching signal V 3 . The third inverter can invert the third switching signal V 3  to generate a fourth switching signal V 4 . The output of the third inverter can be directed to the gate of the sixth switching FET  529  (to provide the fourth switching signal V 4  to regulate the sixth switching FET  529 ), and to the input of a second divide by two frequency divider  555 . The second divide by two frequency divider may convert the fourth switching signal V 4  into a disable signal  533  that is routed to the gate of the disable FET  532  in the current mirror  530 . 
     Switching FETs in a given CML book receive switching signals generated on different sides of an inverter in the sequencing circuit  550  and that have the same signal transition frequency. The first level  521  of the switching circuit  520  is configured, according to embodiments, to receive switching signals that undergo signal transitions with the same frequency as the clock signal generated by the clock signal generator  540 . The third and fourth switching signals that are received by the second CML book  592 , have a transition frequency that is half that of the clock signal and the first and second switching signals V 1  and V 2 . The disable signal  533 , being transmitted to the gate of the disable FET  532  from the output of second divide by two frequency divider  555  has a signal transition frequency that is half that of the third and fourth switching signals, and one fourth that of the first and second switching signals. 
       FIG. 6  shows a waveform table  600  for an integrated circuit, having at least four branches, that is connected to a current regulating circuit, according to embodiments. The voltages, currents, clock signals, and clock signal transitions associated with the waveform table  600  are generally consistent with embodiments of current regulating circuits similar to those described in  FIG. 5 . Structural elements described hereafter are intended to describe circuit elements similar to those shown in  FIG. 5 , to describe positions in such circuits where currents and voltages can be measured, and to describe the behavior of such circuits in response to the sequencing signals that they generate during operation. References made herein to  FIG. 5  are intended to be interpreted broadly to all embodiments that have two-level switching circuits and perform the regulating function described herein. 
     A first switching signal  691  (V 1 ,  FIG. 5 ), a second switching signal  692  (V 2 ,  FIG. 5 ), a third switching signal  693  (V 3 ,  FIG. 5 ), and a fourth switching signal  694  (V 4 ,  FIG. 5 ) are sequencing signals generated by the sequencing circuit that control the performance of the current regulating circuit. A disable signal  681  is also a sequencing signal generated by the sequencing circuit that regulates the operation of a current source FET in a clock mirror of the current regulating circuit. A clock signal  610 , transmitted from a clock signal generator to a sequencing circuit, can be transformed into sequencing signals by the sequencing circuit in order to coordinate the operation of the current regulating circuit. A bias voltage V(bias)  680  can be measured at a gate of a current source FET such as current source FET  531 . Branch electrical currents I 1   671 , I 2   672 , I 3   673 , and I 4   674  can each be measured at a point along an active electrical pathway between the current source and the respective branch of an integrated circuit that is connected to the current regulating circuit. 
     A first clock cycle  611  begins with clock signal transition  620  and ends with clock signal transition  622 . A second clock cycle  612  of clock signal  610  begins with clock signal transition  622  and ends with clock signal transition  624 . During the first clock cycle  611  and the second clock cycle  612 , the disable signal  681  has a low voltage. 
     A first regulatory cycle  615  begins at clock signal transition  620  and ends at clock signal transition  628 , having spanned four clock cycles and eight clock signal transitions. During the first half of the first regulatory cycle  615 , the disable signal  681  has a low voltage, and in the second half (ending at clock signal transition  628 ) the disable signal  681  has a high voltage. During the first half of the first regulatory cycle  615 , corresponding to first clock cycle  611  and second clock cycle  612 , and in embodiments such as are described by  FIG. 5 , electrical current can flow through a current source FET  531 , while in the second half of the first regulatory cycle  615 , the current source FET  531  cannot pass an electrical current because the disable FET  532  has been activated. According to embodiments, the duty cycle of electrical current flowing through the current source FET  532  is approximately 50% (4/8) of the first regulatory cycle  615 . As electrical current can flow through a branch of the integrated circuit  510  during half of a clock cycle, the duty cycle of electrical current flowing through any one branch of the integrated circuit  510  is approximately 12.5% (1/8) of the first regulatory cycle  615 . 
     The clock signal  610  generated by a clock signal generator such as a clock signal generator  540  can be converted by a sequencing circuit  550  into a plurality of switching signals and a disable signal  681 . More particularly, clock signal  610  can be converted into a first switching signal  691  by passing through a first inverter  551 , and the first switching signal  691  can be converted into a second switching signal  692  by being passed through a second inverter  552 . First switching signal  691  and second switching signal  692  share the same signal transition frequency as the clock signal  610 , changing between high and low voltage states with consecutive clock signal transitions:  620 ,  621 ,  622 ,  623 ,  624 ,  625 ,  626 , and  627 . 
     The third switching signal  693  and the fourth switching single  694  change between high and low voltage states with alternate clock signal transitions, such as  620 ,  622 ,  624 , and  626 . This lowered transitional frequency results from the third and fourth switching signals, being drawn from the sequencing circuit  550  after the signal passes through a divide by two frequency divider  553 . The third switching signal  693  can exit the sequencing circuit  550  at a point connected to the output of the divide by two frequency divider  553 , while the fourth switching signal  694  can be generated (in an opposite phase to the third switching signal  693 ) by passing the third switching signal  693  through a third inverter  554  in the sequencing circuit  550 . 
     A disable signal  681  can be generated within the sequencing circuit  550  by passing the fourth switching signal  694  through a second divide by two frequency divider  555 . The disable signal  681  can be routed to the gate of a disable FET  532  in order to periodically prevent the flow of electrical current through a current source FET  531 , putting the current regulating circuit and the integrated circuit  510  into a resting state during burn-in testing. 
     A hierarchy of control of sequencing signals on a current regulating circuit can be described as follows. First, the state of the disable signal  681  determines whether current source FET will be activated: when the disable signal  681  has a high voltage, the current source FET will be deactivated (and the disable FET will be activated); and when the voltage is low, electrical current can flow through the current source FET, some pathway through the switching circuit  520 , and some branch of the integrated circuit  510 . The duty cycle of the disable FET allowing electrical current to flow is approximately 50% (4/8 of the first regulatory cycle  615 ). Other current flow duty cycles are envisioned, and may be implemented, according to the design characteristics and performance requirements of the integrated circuits that are tested using burn-in testing. The numerical values of current flow duty cycle described herein are merely indicative of the characteristics of the specific embodiments to which they refer, and are not intended to limit the scope of the present disclosure. The disable signal  681  and the bias voltage V(bias)  680 , undergo signal transitions at one fourth the frequency of the clock signal  610 . 
     Secondly, the third and fourth switching signals  693  and  694 , respectively, activate one side or the other of the third CML book  593 . When the third switching signal  693  activates the fifth switching FET  528 , electrical current can flow through some side of the first CML book  591 . When the fourth switching signal  694  activates the sixth switching FET  529 , electrical current can flow through some side of the second CML book  592 . The duty cycle of one side of the third CML book flowing electrical current through a branch of the integrated circuit is approximately 25% (2/8 of the first regulatory cycle  615 ). The third and fourth switching signals  693  and  694 , respectively, undergo signal transitions at one half the rate of the clock signal  610 . 
     Thirdly, the first switching signal  691  and the second switching signal  692  are configured to enable a side of either the first CML book  591  or the second CML book  592 , respectively. When the first switching signal  691  has a high voltage, the first switching FET  524  and the third switching FET  526  can be activated, and electrical current can pass through one of them according to the status of the third and fourth switching signals  693  and  694 , respectively. Similarly, the second switching signal  692  can activate both the second switching FET  525  and the fourth switching FET  527  at the same time, and electrical current can flow through one of them according to the status of the third and fourth switching signals  693  and  694 , respectively. The first and second switching signals  691  and  692 , respectively, undergo signal transitions at the same frequency as the clock signal  610 . 
     A first branch electrical current I 1   671  can flow through the first branch  511  of the integrated circuit  510  when the first switching signal  691 , the third switching signal  693 , and bias voltage V(bias)  680  have high voltages. Upon a clock signal transition such as clock signal transition  621 , the second switching signal  692  can transition to a high voltage and the second branch electrical current I 2   672  can flow. Upon a clock signal transition such as clock signal transition  622 , the first switching signal  691  and the fourth switching signal  694  transition to a high voltage, allowing the third branch electrical current I 3   673  to flow, and upon a clock signal transition such as clock signal transition  623  the second switching signal  692  can transition to a high voltage, allowing the fourth branch electrical current I 4   674  to flow. 
     The descriptions of the various embodiments of the present disclosure have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Various embodiments that include different numbers and locations of inverters, CML books, current control logic circuits, and other logical gates connected to switching FETs, or in the current mirror, or to the disable FET can produce embodiments that also demonstrate the breadth and scope of the present disclosure. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.