Abstract
Temperature sensors are routinely found in devices used to monitor the environment, the human body, industrial equipment, and beyond. In many such applications, the energy available from batteries or the power available from energy harvesters is extremely limited due to limited available volume, and thus the power consumption of sensing should be minimized in order to maximize operational lifetime. Here we present a new method to transduce and digitize temperature at very low power levels. Specifically, two pA current references are generated via small tunneling-current metal-oxide-semiconductor field effect transistors (MOSFETs) that are independent and proportional to temperature, respectively, which are then used to charge digitally-controllable banks of metal-insulator-metal (MIM) capacitors that, via a discrete-time feedback loop that equalizes charging time, digitize temperature directly. The proposed temperature sensor was integrated into a silicon microchip and occupied 0.15 mm2 of area. Four tested microchips were measured to consume only 113 pW with a resolution of 0.21 °C and an inaccuracy of ±1.65 °C, which represents a 628× reduction in power compared to prior-art without a significant reduction in performance.
Highlights
External CWT frequency sources for digitization that are not included in the quoted power number
We present a new temperature sensing technique that relies on complementary temperature dependencies of n- and p-type metal-oxide-semiconductor field effect transistors (MOSFETs) biased in the subthreshold region, together with CWT tunneling currents and a capacitive charging-time-to-digital feedback architecture that digitizes temperature at 113 pW in a fully monolithically-integrated manner, which represents a 628× reduction in power over prior-art[16]
When biased in the subthreshold or weak-inversion regime (i.e., |Vgs < Vth| where Vgs is the gate to source voltage and Vth is the threshold voltage of the transistors), the drain current of each transistor is given by: Isub where μ is mobility, Cox is oxide capacitance, W and L are the transistor width and length, respectively, n is subthreshold slope factor, φT is thermal voltage, and Vds is the drain to source voltage
Summary
External CWT frequency sources for digitization that are not included in the quoted power number. We present a new temperature sensing technique that relies on complementary temperature dependencies of n- and p-type MOSFETs biased in the subthreshold region, together with CWT tunneling currents and a capacitive charging-time-to-digital feedback architecture that digitizes temperature at 113 pW in a fully monolithically-integrated manner, which represents a 628× reduction in power over prior-art[16]. A 2-transitor (2T) subthreshold PTAT voltage reference generator[16] (VRG) was implemented to serve as the temperature sensing element, while another temperature-stabilized 2T subthreshold VRG18 was employed as a CWT reference, replacing conventionally power-hungry band-gap VRGs. The PTAT and CWT analog voltages were converted to pA-level currents via self-biased current generators based on tunneling effects. Temperature was digitized by charging digitally-controllable monolithic MIM capacitors with the pA-level currents and matching the charging time between the PTAT and CWT paths via feedback-driven tuning of the MIM capacitors for direct ultra-low-power digital readout
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