A 200mV Operable On-Chip Temperature Sensor for IoT Devices Powered by Energy Harvesters with Ultra-Low Output Voltage

This paper describes a 24.8 μm2 on-chip CMOS digital temperature sensor core operable on 95 nW at the 200mV supply voltage. The target of this sensor is IoT devices powered by energy harvesters with ultra-low output voltage such as thermoelectric generators. Unlike the conventional ring-oscillator type temperature sensor, the proposed sensor directly monitors the time to store the electrical charge into the capacitance by temperature-dependent leakage current. The result of this 'temperature-to-time' conversion is given to a counter to obtain a digital output. We designed and fabricated the proposed temperature sensor circuit in a 65 nm FDSOI technology. Measured results demonstrated that the proposed temperature sensor successfully operated at 0.2 V - 1.2 V for the temperature range from -20 °C to 80 °C. Excellent linearity in the relationship between the logarithm value of the counter output and the reciprocal of absolute temperature was demonstrated in an analytical model and confirmed in the measured results. This enables us to identify the temperature from the counter output by a 2-point calibration with good accuracy. Since all MOS transistors for the temperature-to-time conversion operate in the subthreshold region (i.e. at the off-state) in 0.2 V, the accuracy is significantly affected by the process variation. In the proposed temperature sensor, by tuning the body-bias voltage to pMOS (V BS,p) and the reference voltage (VREF), the maximum error in the temperature across four fabricated sensors on a chip was reduced to 0.24X. By tuning V BS,p and VREF to the conversion temperature range [-20 °C, 80°C], the error was improved to +5.7 °C/-5.1 °C, while further improved to +1.2 °C/-2.4 °C by tuning them to the temperature range [0 °C, 80°C].