Weimen Li

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IntroductionAs sensing systems have grown in popularity and pervasiveness, minimizing their power consumption has become critical. Low power consumption increases device longevity and allows for more versatile applications - from industrial, medical, and automotive systems. One particularly pressing application, forest fire detection, necessitates robust and low-maintenance monitoring solutions. However, existing solutions, such as camera towers and aerial robots, are often exceedingly expensive and ineffective. Aiming to address this problem, we present a present a 60 nW reprogrammable temperature-activated switch that utilizes sub-threshold MOSFET design. It is operational over a temperature range of 3 C to 250C, and consumes 60 nW of power when set to a switching threshold of 100 C. With an exceedingly low operating power, this solution not only enables long-lasting forest fire detection but it also serves an effective alternative to standard temperature sensors for a myriad of applications.MethodologyOur original intention was to design a low-power switch that takes a 10 mV signal as input to switch a standard 1.2 V output using sub-Vt MOSFETs. The fundamental idea behind our approach was to cascade several common-source amplifiers together, but with the input transistor operating in sub-threshold. Since the output of each stage is exponential according to the equation \(\(V_{out}=R\left(\frac{W}{L}\right)I_s\exp\left(\frac{V_{gs}}{nVth}\right)\), each successive stage would take as input the exponential output of the stage before it, such that the final output attains an extremely steep exponential slope which looks much like the desired step output. Due to the \(\(R\times\left(\frac{W}{L}\right)I_s\) factor in the equation, sub-microamp currents required unattainable resistances on the order of several \(\(M\Omega\) in order to function properly. Hence, we replaced the resistors with long-channel MOSFETs operating in deep-triode, which effectively simulated a resistive load. Simulation results of this circuit revealed, much to our surprise, that these deep-triode resistors included substantial current leakage through the gate oxide, on the order of several dozen nA. Therefore, we also replaced these with nominal 2.5V MOSFETs, which reduced the current leakage to the order of fA. Simulation of the circuit at this stage showed that although excellent switching behavior was attained as predicted, the circuit was also extremely sensitive to temperature and process variations, to the extent that variation of a few degrees centigrade, or a different process corner, rendered the circuit nonfunctional. This was very disturbing, for a temperature-sensing circuit whose own behavior is dramatically affected by the temperature is useless. However, further investigation revealed, much to our surprise, that this circuit switch based off of temperature as well as it did switching from an input voltage. Furthermore, we found that tuning the input voltage, which we henceforth refer to as the bias voltage \(\(V_b\), allowed the switch's threshold temperature to be tuned as well. RobustnessAs with many sub_Vt designs, our temperature switch is subject to some degree of process variation. We find that different process corners affect the temperature vs. bias voltage relationship, as well as the maximum temperature range of the circuit. Thankfully, the existence of a settable input bias voltage simultaneously allows for post-fabrication tuning, sNevertheless, we find that the primary functionality of the circuit, as well as the switching behavior, remains functional across all process corners.Results Quiescent Power @ T= 0 C, V_t=125 °C 55 nW V_OH V_OL Temperature Range In the final implementation of our circuit, we achieve <100 nW power consumption for a switching temperatures above 60 C, with 45 nW power consumption when set to the maximum temperature allowed by military-grade components of 125 C. Switching typically occurs over the span of 1 C, with typical worst-case \(\(V_{OL}<0.005\times V_{DD}\) and \(\(V_{OH}>0.98\times V_{DD}\).