Patent Publication Number: US-2022239254-A1

Title: Temperature compensated oscillators and associated methods

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to semiconductor devices, and more particularly relates to temperature compensated oscillators and associated methods. 
     BACKGROUND 
     Semiconductor devices are widely used to process and/or store information related to various electronic devices such as computers, wireless communication devices, cameras, digital displays, and the like. Information is stored by programing different states of a memory cell. Various types of semiconductor memory devices exist, such as non-volatile memory devices (e.g., NOR flash memory devices, 3-dimensional NAND flash memory devices, etc.) and volatile memory devices (e.g., dynamic RAM (DRAM), synchronous dynamic RAM (SDRAM), etc.). 
     Improving memory devices, generally, may include increasing memory cell density, increasing read/write speeds or otherwise reducing operational latency, increasing reliability, increasing data retention, reducing power consumption, reducing areas occupied by integrated circuitry, or reducing manufacturing costs, among other metrics. One way of reducing manufacturing costs is to improve manufacturing processes to increase the margin of successfully manufactured devices. Manufacturers can improve the manufacturing margin by implementing processes that, for example, increase the consistency or tolerance of manufacturing steps (e.g., removal or deposition of materials), improve the scale of manufacturing, reduce variability among memory cells, etc. Further, manufacturers can minimize any impedance mismatch between a memory controller and the memory devices within a system by calibrating input/output impedances of the memory devices to accomplish data rates desired for robust system performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure. The waveforms in the drawings are not necessarily to scale. Instead, emphasis is placed on illustrating clearly the principles of the present disclosure. 
         FIG. 1A  is a block diagram illustrating an oscillator, and  FIG. 1B  illustrates waveforms of signals associated with the oscillator. 
         FIG. 2  is a circuit diagram of an inverter in accordance with embodiments of the present disclosure. 
         FIG. 3A  is a block diagram of an oscillator in accordance with embodiments of the present disclosure, and  FIG. 3B  illustrates waveforms of signals associated with the oscillator. 
         FIG. 4  is a circuit diagram of a resistor in accordance with embodiments of the present disclosure. 
         FIG. 5  is a flowchart illustrating a method of operating an oscillator in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Semiconductor devices delivering high performance (e.g., high bandwidth, low power consumption) may include oscillators that generate clock signals to precisely control various functional circuit blocks. In some embodiments, oscillators provide clock signals to a state machine of circuitry designed to match input/output impedances (ZQ) to a specific value—e.g., during an initialization of a DRAM device. The circuitry may be referred to as a ZQ block that facilitates different combinations of circuit components (e.g., resistors, transistors, etc.) matching the specific ZQ value (e.g., calibrating the ZQ value to 240 Ohms). Moreover, the circuit components of the ZQ block may include similar structures (e.g., legs of a transistor), each having similar resistance/impedance values. Maintaining periods of the clock signals provided to the ZQ block within a specified range across various operating temperatures of the semiconductor devices would be beneficial to improve accuracies of the ZQ calibration and to provide superior noise immunity against switching of input/output pins (DQs) that may occur during the ZQ calibration. 
     Such oscillators may include a resistor (R) and a capacitor (C) forming an RC network, a node of which can be charged to a first voltage at time T 0  (e.g., the time T 0  depicted in  FIG. 1B ). Subsequently, the node can be discharged to a second voltage at time T 1  (e.g., the time T 1  depicted in  FIG. 1B ), which in turn, would cause an inverter connected to the node to trigger (trip or reverse its output). The duration between T 0  and T 1  may be related to a period of the clock signal that the oscillator generates. In some embodiments, the resistor may be formed in a substrate—e.g., a silicon substrate, in which the oscillator is formed. Further, the resistor may be doped (e.g., using diffusion process steps) with various dopant species (e.g., arsenic, phosphorus, boron, indium, etc.) to achieve a desired resistance. Such resistors may be referred to as diffusion resistors. The diffusion resistors may have a positive temperature coefficient—e.g., the resistance increases as the operating temperature increases. The change in the resistance as a function of the operating temperature may result in significant variations over the specified (allowed) operating temperature range for the semiconductor devices (e.g., approximately 25% variations in certain embodiments), which in turn, would result in variations in the periods of the clock signals that the oscillator generates. 
     To address the foregoing challenges, embodiments of the present disclosure provide an inverter with a variable load that is configured to provide different conductance values for the inverter. For example, the variable load includes two or more branches (paths, legs) in parallel, where each branch has a unique conductance value different from each other. Further, the variable load can be coupled to a temperature sensor that generates signals based on determining (sensing) an operating temperature. The signals of the temperature sensor can be provided to the variable load of the inverter such that one or more branches of the variable load can be activated based on the operating temperature. In this manner, the variable load can have different conductance values as a function of the operating temperature such that the inverter can trigger at different voltage levels of the node. As a result of modifying the conductance value of the variable load of the inverter based on the operating temperature, variations in the period of the clock signal can be reduced. 
     By way of example, the oscillator may be trimmed (tuned, calibrated) to generate a clock signal with a target period during testing procedures. The testing procedures may be carried out at a specific temperature (e.g., at 90 degrees Celsius (° C.)), which may be referred to as a probe temperature. To this end, the temperature sensor may be configured to generate a first set of signals (tailored to 90° C.±10%, 90° C.±20%, or the like) to the variable load such that the oscillator can generate the clock signal with the target period. Further, the testing procedure may assess variations in the period of the clock signal by varying the operating temperature such that the temperature sensor can be preconfigured to provide different sets of signals to the variable load at different operating temperatures in order to reduce variations in the period of the clock signal. 
     During the operation, if the operating temperature (e.g., 130° C.) is greater than the probe temperature, the temperature sensor provides a second set of signals (tailored to 130° C.±10%, 130° C.±20%, or the like) to the variable load such that the conductance value of the variable load may be increased to reduce deviations in the period of the clock signal with respect to the target period. Similarly, if the operating temperature (e.g., −40° C.) is less than the probe temperature, the temperature sensor provides a third set of signals (tailored to −40° C.±10%, −40° C.±20%, or the like) to the variable load such that the conductance value of the variable load may be decreased to maintain the period of the clock signal close to the target period. As such, the period of the clock signal that the oscillator generates can be maintained with a predetermined range (as required by a specification) over the allowed operating temperatures of the semiconductor devices. 
       FIG. 1A  is a block diagram schematically illustrating an oscillator  101  of a semiconductor device (e.g., a DRAM device). In the example embodiment of  FIG. 1A , the oscillator  101  includes a resistor  110 , a capacitor  120 , a pull-up component  125 , a first inverter  130 , a feedback pull-down component  150 , all of which are coupled to a node  115 —i.e., the node  115  is common to (shared by) the above identified components. The node  115  may be referred to as an RC node, or a discharge node. Further, the oscillator  101  includes a second inverter  160  coupled to the resistor  110 , as well as an amplifier  135  coupled to the first inverter  130 . The oscillator  101  has an input terminal  105  (which corresponds to the input terminal of the second inverter  160 ) and an output terminal  106  (which corresponds to the output terminal of the amplifier  135 ). Moreover, the oscillator  101  includes a first feedback loop  145  (which may include a first feedback component  146 ) between the amplifier  135  and the feedback pull-down component  150 , and a second feedback loop  155  (which may include a second feedback component  156 ) between the output terminal  106  and the input terminal  105  (and the pull-up component  125 ). 
     As depicted in  FIG. 1A , the resistor  110  has a first terminal connected to the node  115 . In some embodiments, the resistor  110  includes a diffusion resistor. Further, the capacitor  120  is connected to the node  115 . In some embodiments, the capacitor  120  includes two or more metal-oxide semiconductor field-effect transistors (which may also be referred to as MOSFETs or MOS transistors) connected in parallel with each other—e.g., p-channel MOS transistors, n-channel MOS transistors. The resistor  110  and the capacitor  120 , in combination, may be regarded as an RC network of the oscillator  101 . 
     Further, the pull-up component  125  has a first terminal connected to the node  115  and a second terminal connected to a voltage source (Vcc) having a first voltage, V 1  (e.g., the voltage V 1  depicted in  FIG. 1B ). The pull-up component  125 , when activated, is configured to couple the node  115  to Vcc (e.g., at time T 0  described with reference to  FIGS. 1B ) to charge the node  115  to V 1 . The pull-up component  125 , when deactivated, is configured to decouple (disconnect) the node  115  from Vcc—e.g., after charging the node  115  to V 1 . In some embodiments, the pull-up component  125  includes a p-channel MOS transistor with its gate coupled to the output terminal  106  of the oscillator  101  (through the second feedback loop  155 ). In this regard, the p-channel MOS transistor includes a gate coupled to an output terminal of the amplifier  135 , a source coupled to the voltage source (Vcc), and a drain coupled to the node  115 . 
     The first inverter  130  may include a complementary metal-oxide-semiconductor (CMOS) inverter having an n-channel MOS (nMOS) transistor and a p-channel MOS (pMOS) transistor. Such a CMOS inverter reverses its output in response to a voltage applied to its input. For example, if the voltage applied to its input is less than a trigger voltage of the CMOS inverter, the output of the CMOS inverter may be approximately equal to a voltage (e.g., V 1 ) of the voltage supply (e.g., Vcc), under which the CMOS inverter operates. Similarly, if the voltage applied to its input is equal to and/or greater than the trigger voltage, the output of the CMOS inverter may be approximately equal to a ground potential (a ground, a ground level, a ground node), to which the CMOS inverter is coupled (e.g., Vss). Further, the inverters included in the oscillator  101  (e.g., the first and second inverters  130  and  160 , the inverters  141 - 143  of the amplifier  135 ) may operate under the voltage supply (Vcc). 
     The amplifier  135  may include one or more inverters. In the example embodiment of  FIG. 1A , the amplifier  135  is depicted to include three inverters, namely inverters  141 , 142 , and  143 . Each inverter of the amplifier  135  may correspond to the CMOS inverter described herein. The amplifier  135  is coupled to the first inverter  130  to receive the output (e.g., a voltage waveform) that the first inverter  130  generates. Moreover, the amplifier  135  can be configured to refine the voltage waveform to generate the clock signal of the oscillator  101  at the output terminal  106 . The output terminal  106  of the oscillator  101  (corresponding to the output terminal of the amplifier  135 ) may be coupled to a frequency divider (not shown) to reduce the frequency of the clock signal (to “slow down” the clock signal), prior to feeding the “slowed” clock signal to other functional blocks of the semiconductor device—e.g., the ZQ block. 
     In some embodiments, the feedback pull-down component  150  may include an n-channel MOS transistor with its gate coupled to one of the internal nodes of the amplifier  135  (e.g., a node  136  corresponding to the output terminal of the inverter  142 ). As described in more detail herein with reference to waveforms of  FIG. 1B , the feedback pull-down component  150  may be configured to assist the voltage of the node  115  to reach the ground level (e.g., Vss) after the voltage of the node  115  reaches a trigger voltage of the first inverter  130  at time T 1 . 
     In some embodiments, the second inverter  160  includes the CMOS inverter described herein. The second inverter  160 , when activated, may establish a leakage path  165  for the node  115 . Further, the second inverter  160 , when deactivated, may isolate the node  115  from the ground (e.g., Vss)—e.g., shutting the leakage path  165  while the node  115  is being charged to V 1 . 
     Aspects of the oscillator  101  operation is described herein with reference to  FIG. 1B  in view of various components described with reference to  FIG. 1A . The waveforms and operations of various components of the oscillator  101  are described based on operating principles of digital electronic circuits and functional blocks having two distinct levels of voltages. For example, such voltages may include a “high” voltage (V H ) designated to a first logic state of “1” or “high” and a “low” voltage (V L ) designated to a second logic state of “0” or “low.” The first voltage level may correspond to 0.8 V, 1.0 V, 1.2 V, or any other suitable values to adequately operate the digital electronic circuits and functional blocks. The second voltage level may correspond to a ground potential (a ground, a ground node) for the semiconductor device, typically corresponding to 0 V (or any other suitable values). Further, the oscillator  101  may include other components and/or functional blocks, which are omitted in  FIG. 1A  to avoid obfuscating the principles of the present disclosure. Although foregoing example circuits and functional blocks are described with two logic states or voltage levels, the present disclosure is not limited thereto. For example, the principles of the present disclosure would be applicable to circuits and functional blocks designed to operate with more than two levels of logic states or voltage levels. 
       FIG. 1B  illustrates voltage waveforms (e.g., waveforms  170  and  180 ) corresponding to the voltage of the node  115  as a function of time. The waveforms  170  and  180  illustrate aspects of the oscillator  101  generating clock signals at different operating temperatures. For example, the waveform  170  corresponds to the voltage of the node  115  at a first operating temperature (e.g., a probe temperature of 90° C.). Likewise, the waveform  180  corresponds to the voltage of the node  115  at a second operating temperature, which may be less (lower) than the first temperature—e.g., a temperature corresponding to a minimum allowed operating temperature, for example, −40° C. 
     At time T 0 , the waveform  170  reaches V 1  (i.e., the voltage of the node  115  is charged to V 1 )—e.g., by activating the pull-up component  125  that couples the node  115  to the voltage supply Vcc. As described above, the pull-up component  125  may include a pMOS transistor with its gate coupled to the output terminal  106  of the oscillator  101 , which can provide V L  to the gate of the pMOS transistor through the feedback loop  155  (and/or the feedback component  156 ). As such, the pMOS transistor of the pull-up component  125  may be activated to couple the node  115  to the voltage supply Vcc at T 0 . Further, the input of the second inverter  160  (e.g., a CMOS inverter) can also be provided with V L  through the feedback loop  155  (and/or the feedback component  156 ). Accordingly, the second inverter  160  can facilitate the node  115  to be charged to V 1 —e.g., by shutting the discharge path  165 . 
     Subsequently, the pull-up component  125  may decouple the node  115  from Vcc, and the voltage of the node  115  begins to decrease from V 1 —e.g., due to the leakage current flowing through the leakage path  165  from the node  115 . In this regard, the second inverter  160  may provide the discharge path  165  available after the node  115  has been charged to V 1  and/or been disconnected from Vcc. At time T 1 , the voltage of the node  115  reaches a trigger voltage (V_Trigger) of the first inverter  130 . As described above, the first inverter  130  (e.g., a CMOS inverter) is configured to reverse its output in response to the voltage of the node  115  becoming less than the trigger voltage—e.g., V L  present at the input of the first inverter  130 . As a result, the output of the first inverter  130  may correspond to V H , which in turn, establishes the output of the inverter  142  at the node  136  corresponding to V H . As the feedback pull-down component  150  is coupled to the node  136  through the first feedback loop  145 , V H  at the node  136  may activate the feedback pull-down component  150  (e.g., an nMOS transistor) such that the feedback pull-down component  150  can assist the voltage of node  115  to reach Vss at time T 2 . 
     In response to the voltage of the node  115  reaching Vss at time T 2 , the output of the amplifier  135  (e.g., the output of the inverter  143 ) corresponds to V L , which in turn, activates the pull-up component  125  (and causes the second inverter  160  to shut the discharge path  165 ). In this manner, the pull-up component  125  can charge the node  115  such that the voltage of the node  115  can reach V 1  at time T 3  to repeat the sequence of operations described above. As such, the oscillator  101  may have the node  115  of the RC network to periodically alternate between V 1  and Vss, with a period P 1  (e.g., duration between T 0  and T 3 ). The oscillator  101  may be designed (e.g., by selecting resistance and capacitance values for the RC network) to generate a clock signal with a target period at the probe temperature, which is related to the period P 1 . 
     Behavior of the RC network of the oscillator  101  (and/or the voltage of the node  115 ) may be characterized with a time constant (t) related to a product of the resistance value of the resistor  110  and the capacitance value of the capacitor  120 . For example, the greater the time constant t, the less the rate of voltage change of the node  115  as a function of time (which may also be referred to as dv/dt). In some embodiments, the resistance value of the resistor  110  may vary as a function of the operating temperature (e.g., having a positive temperature coefficient) resulting in the time constant τ of the RC network to vary depending on the operating temperature. For example, if the resistance value of the resistor  110  decreases as the operating temperature decreases, the time constant τ may decrease such that the rate of voltage change would increase—e.g., the RC network discharges relatively fast to reach V_Trigger from V 1 . Similarly, if the resistance value of the resistor  110  increases as the operating temperature increases, the time constant τ may increase such that the rate of voltage change would decrease—e.g., the RC network discharges relatively slowly to reach V_Trigger from V 0 . 
     For example, the waveform  180  illustrates the voltage at the node  115  at a second operating temperature less than the first temperature (e.g., −40° C.). At time T 0 , the waveform  180  reaches V 1  (i.e., the voltage of the node  115  is charged to V 1 )—e.g., by activating the pull-up component  125  that couples the node  115  to the voltage supply Vcc. Subsequently, the voltage of the node  115  begins to decrease (discharge) from V 1  toward V_Trigger—e.g., due to the leakage current flowing through the leakage path  165  from the node  115 . As the resistance of the resistor  110  is reduced (i.e., the time constant τ is smaller, the dv/dt is greater), the slope of the waveform  180  is steeper than that of the waveform  170 . Hence, at time T 4 , which is less than T 1 , the voltage of the node  115  reaches the trigger voltage (V_Trigger) of the first inverter  130 . Thereafter, the voltage of the node  115  reaches Vss (e.g., through the feedback pull-down component  150 ), and then re-charges to V 1  at time T 5 . As such, the oscillator  101  may have the node  115  of the RC network to periodically alternate between V 1  and Vss, with a period P 2  (e.g., duration between T 0  and T 5 ) that is less than the period P 1 . 
     Accordingly, the oscillator  101  may generate a clock signal with a shorter period than the target period if operating at a lower temperature than the probe temperature. Similarly, the oscillator  101  may generate a clock signal with a longer period than the target period if operating at a higher temperature than the probe temperature. Such variations in the period of the clock signal that the oscillator  101  generates may be disadvantageous for functional blocks (e.g., ZQ blocks) to perform accurately based on receiving the clock signal from the oscillator  101 . 
       FIG. 2  is a circuit diagram of an inverter  201  in accordance with embodiments of the present disclosure. The inverter  201  includes an nMOS transistor  210  and a pMOS transistor  215  connected to the nMOS transistor at an output terminal of the inverter  201 . Gates of the nMOS transistor  210  and the pMOS transistor  215  are connected to form an input terminal of the inverter  201 . Further, the inverter  201  includes a variable load  220  including a first terminal  221  connected to a voltage source (e.g., Vint in  FIG. 3A ) and a second terminal  222  connected to the pMOS transistor  215 . The variable load  220  may be configured to provide two or more conductance values for the inverter  201  as a function of an operating temperature of the inverter  201 . 
     In some embodiments, the variable load  220  includes a set of conductive paths (branches), each one of the conductive paths disposed between the first and second terminals  221  and  222 . Further, individual conductive paths of the variable load  220  are coupled to corresponding terminals of a temperature sensor configured to sense an operating temperature of the inverter  201  (or the oscillator  301  including the inverter  201 ). The temperature sensor may generate signals to activate one or more conductive paths of the variable load  220  in response to sensing the operating temperature of the inverter  201 . Moreover, the individual conductive paths may have different conductance values from each other. In some embodiments, the variable load  220  includes a plurality of MOS transistors (e.g., pMOS transistors  225   a/b/n  as depicted in  FIG. 2 ) disposed between the first and second terminals  221  and  222 . Further, gates of the MOS transistors (e.g., gates  226  of the pMOS transistors) are coupled to corresponding output terminals of the temperature sensor. As such, the temperature sensor can activate one or more MOS transistors of the variable load  220  in response to sensing the operating temperature of the inverter  201 . 
     The MOS transistors (e.g., pMOS transistors) of the variable load  220  may be configured to have different sizes (e.g., channel lengths (L) and/or channel widths (W)) that determines different conductance values of the MOS transistors. For example, the pMOS transistor  225   a  has W equal to 1.76 micro meters (μm) and L equal to 1 μm, and the pMOS transistor  225   b  has W equal to 0.11 μm and L equal to 8 μm. In other words, the pMOS transistor  225   a  has a greater conductance than the pMOS transistor  225   b . In some embodiments, a conductive path of the  220  includes a single MOS transistor (e.g., the pMOS transistors  225   a/b/n ). Although,  FIG. 2  depicts the variable load  220  to include pMOS transistors, the present disclosure is not limited thereto. For example, the variable load  220  may include nMOS transistors, bipolar junction transistors (BJTs), junction field effect transistors (JFETs), or other suitable components that can be activated or deactivated to provide different conductance values of the variable load  220 . 
     In some embodiments, a conductive path of the variable load  220  includes two or more MOS transistors that are serially connected to each other between the first and second terminals  221  and  222 —e.g., a conductive path  227 . Such a conductive path may have different configurations of the two or more MOS transistors as shown in the example conductive paths  227   a  and/or  227   b . Further, at least one MOS transistor (e.g., the pMOS transistors  225   i _ 1 ,  225   i _ 2 ) of the two or more MOS transistors has a gate (e.g., the gate  226   i _ 1  or  226   i _ 2  of the pMOS transistor  225   i _ 1  or the pMOS transistor  225   i _ 2 ) coupled to an output terminal of the temperature sensor. Moreover, at least one MOS transistor of the two or more MOS transistors may be configured to be always conductive during operation—e.g., the pMOS transistor  225   i _ 3  with its gate connected to Vss (or a ground node), the pMOS transistor  225   i _ 4  with its gate connected to Vss (or a ground node). 
     As described in more detail with reference to  FIG. 3B , the inverter  201  having the variable load  220  can compensate the variations in the time constant (t) of the RC network of the oscillator  101 . For example, if the resistor  110  has a positive temperature coefficient, at a relatively low operating temperatures (e.g., −40° C.), the temperature sensor may decrease the conductance of the variable load  220 —e.g., activating the pMOS transistor  225   b  (and deactivating the pMOS transistor  225   a , in some cases). In this manner, the inverter  201  may trigger (e.g., reverses its output) at a relatively lower trigger voltage to compensate the reduction in the clock signal period (e.g., P 2  at a lower temperature being less than P 1  at a higher temperature as shown in  FIG. 1B ). Similarly, at a relatively high operating temperatures (e.g., 130° C.), the temperature sensor may increase the conductance of the variable load  220 —e.g., activating the pMOS transistor  225   a  (and deactivating the pMOS transistor  225   b , in some cases) such that the inverter  201  may trigger (e.g., reverses its output) at a relatively high trigger voltage. 
     The temperature sensor may be configured to generate signals to activate (or deactivate) one or more conductive paths of the variable load  220 . By way of example, if the variable load  220  has eight (8) conductive paths, the temperature sensor may generate different combinations of signals (e.g., up to 255 combinations if necessary, in view of the  256  (2 8 ) theoretically possible distinct combinations), where each combination of signals causes the variable load  220  to provide a distinct conductance value for the inverter  201 . As such, the oscillator  101  may be characterized (e.g., during probe procedure and/or test procedure) to assess ranges of the clock signal frequencies that the oscillator  101  may generate as a function of the operating temperature, and the temperature sensor may be preconfigured to generate certain combinations of signals corresponding to sub-ranges of the operating temperature (e.g., −40° C.±10%, −10° C.±10%, 20° C.±10%, 90° C.±10%, 100° C.±10%, 130° C.±10%, etc.) such that the oscillator  101  can generate clock signals having frequencies within a predetermined range across the entire operating temperature range, despite variations in the operating temperatures. 
       FIG. 3A  is a block diagram schematically illustrating an oscillator  301  of a semiconductor device (e.g., a DRAM device) in accordance with embodiments of the present disclosure. The oscillator  301  may include aspects of the oscillator  101  described with reference to  FIG. 1A . For example, the oscillator  301  includes the capacitor  120 , the pull-up component  125 , the feedback pull-down component  150 , the second inverter  160 , and the amplifier  135 . Moreover, the oscillator  301  includes the first feedback loop  145  (which may include the first feedback component  146 ) between the amplifier  135  and the feedback pull-down component  150 , as well as the second feedback loop  155  (which may include the second feedback component  156 ). Further, the oscillator  301  includes the inverter  201  coupled to a temperature sensor  390  (which may be external to the oscillator  301  in some cases) as described with reference to  FIG. 2 , and a resistor  310 . Further,  FIG. 3A  depicts a voltage supply Vint configured to couple with the pull-up component  125 , which may be an internally generated and/or regulated power supply. Further, the inverters included in the oscillator  301  (e.g., the inverter  201  with the variable load  220 , the second inverter  160 , the inverters  141 - 143  of the amplifier  135 ) may operate under Vint. 
       FIG. 3B  illustrates voltage waveforms (e.g., waveforms  370  and  380 ) as a function of time, which correspond to a voltage of the node  315  of the oscillator  301 . For example, the waveform  370  corresponds to the voltage of the node  315  at a first operating temperature (e.g., a probe temperature at 90° C.). Likewise, the waveform  380  corresponds to the voltage of the node  315  at a second operating temperature, which may be less (lower) than the first temperature (e.g., a temperature corresponding to a minimum allowed operating temperature, for example, −40° C.). 
     The oscillator  301  may operate similarly to the oscillator  101  as described with reference to  FIGS. 1A and 1B . For example, the waveform  370  (e.g., at a probe temperature 90° C.) illustrates that at time Ta, the node  315  may be coupled to Vint to reach (e.g., charged) to a second voltage V 2  of Vint. At time Tb, after having been discharged through the discharge path  165 , the voltage of the node  315  reaches a first trigger voltage (V_Trigger 1 ) of the inverter  201 . Subsequently, at time Tc, the waveform  370  depicts the voltage of the node  315  corresponding to Vss (e.g., through the feedback pull-down component  150 ). Thereafter, at time Td, the voltage of the node  315  is charged to V 2  to repeat the operating sequence generating the clock signal at the output terminal  306 , namely having the voltage of the node  315  to periodically alternate between V 2  and Vss, with a period Pa (e.g., duration between Ta and Td) at the first operating temperature (e.g., at a probe temperature 90° C.). 
     As described herein, during the probe procedure, the temperature sensor  390  can be preconfigured to activate one or more conductive paths of the variable load  220  such that the period Pa is optimized (tailored, tuned) for the oscillator  301  to generate the clock signal with the target period (e.g., a target frequency) at the output terminal  306 . Moreover, the temperature sensor  390  can be preconfigured to activate different combinations of the conductive paths of the variable load  220  in response to determining the operating temperatures for the oscillator  301  such that the clock frequencies that oscillator  301  generates can be maintained within a predetermined range across the operating temperatures (e.g., between −40° C. to 130° C.) of the semiconductor device. 
     For example, if the oscillator  301  operates at the second temperature less than the first temperature (e.g., at −40° C.), the time constant of the RC network may decrease as a result of the reduced resistance of the resistor  310  at the second temperature—e.g., when compared to the time constant at the probe temperature of 90° C. As such, the rate of voltage change of the node  315  is increased (i.e., dv/dt at −40° C. is greater than that at 90° C.). If the inverter  201  operates with the same trigger voltage (V_trigger 1 ) as in the first temperature, the inverter  201  would have triggered at time Te, which in turn, would result in a reduced period of the clock signal that oscillator  301  generates. The temperature sensor  390 , however, can reduce the conductance value of the variable load  220  in response to sensing the second temperature by generating a different set of signals to the variable load  220  (to activate a different set of paths of the variable load  220  to reduce the conductance value). 
     As a result, the inverter  201  operates with a different trigger voltage (e.g., V_trigger 2 , which is less than V_trigger 1 ) at the second temperature. The change in the trigger voltage extends (prolongs) the time for the inverter  201  to trigger (reverses its output)—e.g., from Te to Tb (or Tb_ 1  approximately same as Tb, within ±3%, ±5%, ±10%, or the like). Similarly, the inverter  201  may operates with yet another trigger voltage (e.g., V_trigger 3 , not shown, which may be greater than V_trigger 1 ) at a third temperature greater than the first temperature to compensate for the increase in the time constant (i.e., a reduced dv/dt). In this manner, the oscillator  301  can maintain the frequencies of the clock signals it generates within a predetermined range across the operating temperatures (e.g., between −40° C. to 130° C.) of the semiconductor device. 
     In some embodiments, variations in voltage levels of the voltage supply (e.g., Vcc depicted in  FIG. 1A ) cause variations in the clock signal that the oscillator (e.g., the oscillator  101 ) generates. For example, variations in Vcc (e.g., an external voltage supply) may result in changes in the trigger voltage (V_trigger) of the inverter  130 , which in turn, would result in variations in the frequency of the clock signal. As depicted in  FIG. 3A , the oscillator  301  includes Vint (e.g., an internally generated and/or regulated voltage supply) such that the variations in Vint would be reduced than that of an external voltage supply (e.g., Vcc of the oscillator  101 ). In this manner, the oscillator  301  can generate clock signals that are less prone to variations in the voltage levels of the voltage supply. 
     In some embodiments, variations in the fabrication process cause variations in the clock signal that the oscillator (e.g., the oscillator  101 ) generates. For example, variations in patterning process (e.g., photolithography process, etch process, etc.) may result in variations in physical dimensions (e.g., length, width) of the resistor (e.g., the resistor  110 ) of the RC network. The resistor  310  included in the oscillator  301  may be configured to compensate for such process variations as described in more detail with reference to  FIG. 4 . 
       FIG. 4  is a circuit diagram of the resistor  310  in accordance with embodiments of the present disclosure. The resistor  310  includes a set of sub-resistors (R 0 , R 1 , through Rk) connected in series. Further, certain sub-resistors (e.g., R 1  through Rk) are coupled to corresponding MOS transistors (e.g., M 1  M 2 , through Mk) that are configured to electrically bypass (e.g., shunt) one or more sub-resistors of the set, if activated. For example, if none of the MOS transistors are activated, the resistor  310  would have a resistance R corresponding to R 0 +R 1 + . . . +Rk. However, if M 1  is activated, the resistor  310  would have R corresponding to R 0 . Further, if Mj is activated, the resistor  310  would have R corresponding to R 0 +R 1 + . . . +Ri. Further, the increase (or decrease) in the resistance value (ΔR) as one or more sub-resistors are activated (or deactivated) can be maintained to be approximately constant such that the resistor  310  may have a range of resistance with approximately linear incremental (or decremental) steps. In this manner, during the probe procedure, the resistor  310  can be optimized (tuned, tailored) to compensate for the process variations (i.e., at least the variations in the resistance value stemming therefrom) by determining which MOS transistor to activate. 
       FIG. 5  is a flowchart  500  illustrating a method of operating an oscillator in accordance with embodiments of the present disclosure. The flowchart  500  may be an example of or include aspects of a method that an oscillator (e.g., the oscillator  301 ) may perform as described with reference to  FIGS. 1A through 4 . 
     The method includes receiving first input from a temperature sensor indicating that an operating temperature of an oscillator is within a first range of the operating temperatures, where the oscillator includes an inverter having a variable load configured to provide two or more conductance values as a function of the operating temperature (box  510 ). In accordance with one aspect of the present technology, the receiving feature of box  510  can be performed by the oscillator, as described with reference to  FIGS. 1A through 4 . 
     The method further includes generating a first clock signal with a first operating frequency in response to receiving the first input (box  515 ). In accordance with one aspect of the present technology, the receiving feature of box  515  can be performed by the oscillator, as described with reference to  FIGS. 1A through 4 . 
     The method further includes receiving second input from the temperature sensor indicating that the operating temperature is within a second range of the operating temperatures different than the first range of the operating temperatures (box  520 ). In accordance with one aspect of the present technology, the receiving feature of box  520  can be performed by the oscillator, as described with reference to  FIGS. 1A through 4 . 
     The method further includes generating a second clock signal with a second operating frequency in response to receiving the second input, where the first and second operating frequencies are within a predetermined range (box  525 ). In accordance with one aspect of the present technology, the receiving feature of box  525  can be performed by the oscillator, as described with reference to  FIGS. 1A through 4 . 
     In some embodiments, generating the second clock signal with the second operating frequency includes decreasing the conductance value of the variable load if the second range of the operating temperatures is lower than the first range of the operating temperatures, or increasing the conductance value of the variable load if the second range of the operating temperatures is higher than the first range of the operating temperatures. In some embodiments, the variable load includes a first p-channel metal-oxide semiconductor (pMOS) transistor with a first size and a second pMOS transistor with a second size greater than the first size, and generating the second clock signal with the second operating frequency includes activating the first pMOS transistor if the second range of the operating temperatures is lower than the first range of the operating temperatures, or activating the second pMOS transistor if the second range of the operating temperatures is higher than the first range of the operating temperatures. 
     The present disclosure also relates to an apparatus for performing the operations herein. This apparatus can be specially constructed for the intended purposes, or it can include a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program can be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, each coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems can be used with programs in accordance with the teachings herein, or it can prove convenient to construct a more specialized apparatus to perform the method. The structure for a variety of these systems will appear as set forth in the description below. In addition, the present disclosure is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages can be used to implement the teachings of the disclosure as described herein. 
     The present disclosure can be provided as a computer program product, or software, that can include a machine-readable medium having stored thereon instructions, which can be used to program a computer system (or other electronic devices) to perform a process according to the present disclosure. A machine-readable medium includes any mechanism for storing information in a form readable by a machine (e.g., a computer). In some embodiments, a machine-readable (e.g., computer-readable) medium includes a machine (e.g., a computer) readable storage medium such as a read only memory (“ROM”), random access memory (“RAM”), magnetic disk storage media, optical storage media, flash memory components, etc. 
     In the foregoing specification, embodiments of the disclosure have been described with reference to specific example embodiments thereof. It will be evident that various modifications can be made thereto without departing from the broader spirit and scope of embodiments of the disclosure as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. For example, the example resistors included in the oscillators (e.g., the oscillator  101 , the oscillator  301 ) have been described to have a positive temperature coefficient, but the present disclosure is not limited thereto. In other embodiments, the resistors may have a negative temperature coefficient. Additionally, or alternatively, the capacitance value of the capacitor (e.g., the capacitor  120 ) may have significant variability as a function of the operating temperature. In such embodiments, the RC network may behave differently as the operating temperature varies than the foregoing descriptions and illustrations of the present disclosure. Nonetheless, the oscillator and/or the variable load of the inverter  201  (in conjunction with the temperature sensor determining the conductance values of the variable load) may be trained and optimized during the probe procedure such that the oscillator can maintain the frequencies of the clock signals it generates within a predetermined range, across the operating temperatures (e.g., between −40° C. to 130° C.) of the semiconductor device. 
     Those skilled in the art will appreciate that the components, blocks, and steps illustrated in  FIGS. 1A through 5  described above, may be altered in a variety of ways. For example, the order of the logic may be rearranged, substeps may be performed in parallel, illustrated logic may be omitted, other logic may be included, etc. In some implementations, one or more of the components described above can execute one or more of the processes described below. 
     It should be noted that the methods described above describe possible implementations, and that the operations and the steps may be rearranged or otherwise modified and that other implementations are possible. Furthermore, embodiments from two or more of the methods may be combined. 
     Information and signals described herein may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. Some drawings may illustrate signals as a single signal; however, it will be understood by a person of ordinary skill in the art that the signal may represent a bus of signals, where the bus may have a variety of bit widths. 
     The functions described herein may be implemented in hardware, software executed by a processor, firmware, or any combination thereof. Other examples and implementations are within the scope of the disclosure and appended claims. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. 
     Reference in this specification to “implementations” (e.g. “some implementations,” “various implementations,” “one implementation,” “an implementation,” “some embodiments,” etc.) means that a particular feature, structure, or characteristic described in connection with the implementation is included in at least one implementation of the disclosure. The appearances of these phrases in various places in the specification are not necessarily all referring to the same implementation, nor are separate or alternative implementations mutually exclusive of other implementations. Moreover, various features are described which may be exhibited by some implementations and not by others. Similarly, various requirements are described which may be requirements for some implementations but not for other implementations. 
     As used herein, being above a threshold means that a value for an item under comparison is above a specified other value, that an item under comparison is among a certain specified number of items with the largest value, or that an item under comparison has a value within a specified top percentage value. As used herein, being below a threshold means that a value for an item under comparison is below a specified other value, that an item under comparison is among a certain specified number of items with the smallest value, or that an item under comparison has a value within a specified bottom percentage value. As used herein, being within a threshold means that a value for an item under comparison is between two specified other values, that an item under comparison is among a middle specified number of items, or that an item under comparison has a value within a middle specified percentage range. Relative terms, such as high or unimportant, when not otherwise defined, can be understood as assigning a value and determining how that value compares to an established threshold. For example, the phrase “selecting a fast connection” can be understood to mean selecting a connection that has a value assigned corresponding to its connection speed that is above a threshold. 
     As used herein, including in the claims, “or” as used in a list of items (for example, a list of items prefaced by a phrase such as “at least one of” or “one or more of”) indicates an inclusive list such that, for example, a list of at least one of A, B, or C means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). Also, as used herein, the phrase “based on” shall not be construed as a reference to a closed set of conditions. For example, an exemplary step that is described as “based on condition A” may be based on both a condition A and a condition B without departing from the scope of the present disclosure. In other words, as used herein, the phrase “based on” shall be construed in the same manner as the phrase “based at least in part on.” 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Rather, in the foregoing description, numerous specific details are discussed to provide a thorough and enabling description for embodiments of the present technology. One skilled in the relevant art, however, will recognize that the disclosure can be practiced without one or more of the specific details. In other instances, well-known structures or operations often associated with memory systems and devices are not shown, or are not described in detail, to avoid obscuring other aspects of the technology. In general, it should be understood that various other devices, systems, and methods in addition to those specific embodiments disclosed herein may be within the scope of the present technology.