IC Takes Major Step Closer to Zero-Power Digitized Temperature Sensor

1. This near-zero-power temperature sensor/digitizer, which consumes just on 113 picowatts of power, was fabricated in a standard 65-nm CMOS process and has a die size of  0.15 mm².

The sensor/converter uses a standard 65-nm process and has an area of 0.15 mm² (Fig. 1). It operates from −20°C to +40°C, which is wide enough for the target applications, while its one-second update rate is also adequate for most temperature-related situations.

Gate Leakage is a Good Thing?

The design involves two major functional blocks: a pair of current sources and a temperature-to-digital readout. Key to the design is leveraging the gate-leakage effects in transistors for an ultra-low-power current source.

As the gate material of a transistor becomes thinner, this material is no longer able to fully block electrons from leaking through as it can in a larger-geometry device, a phenomenon called the quantum-tunneling effect. The UCSD design takes this gate leakage—normally considered a problem—and uses the associated tiny electron flow to build a current source. This current source is then utilized to create a lower-power approach to digitization.

Note that one common way to digitize temperature (or other sensor output currents) is to pass the current though a temperature-dependent resistor, and then convert the resultant voltage across the resistor. However, that’s a power-hungry approach, so the team instead decided to digitize the temperature directly with a variation on the dual-slope analog-to-digital conversion technique.

2.  Architecture of the proposed temperature sensor: A temperature-stable current source was employed to generate a constant-with-temperature (CWT) ramp voltage, V_ramp , _CWT , by charging capacitor C_REF  (A). A proportional-to-absolute temperature (PTAT) current source, which was employed as the temperature sensing core by converting temperature to a corresponding current, generated a PTAT ramp voltage, V_{ramp, PTAT} , by charging a digitally-controllable bank of capacitors C_DAC  (B). An analog processing unit consisting of a temperature-stabilized VRG, comparators, and an arbiter was implemented to translate the temperature-encoded analog voltages to digital signals (C). Schematic of the Arbiter (D). A digital processing unit (DPU) processes the information, controls C_DAC , and generates the digital codes corresponding to the ambient temperature (E). An example operation of the DPU illustrates that C_DAC was tuned via discrete-time digital feedback to match the rising time of V_ramp, _CWT in the reference sensing unit (RSU) (F).

They built two current sources—one to charge a capacitor for a fixed amount of time independent of the temperature, and the other with a charging rate that’s a function of temperature (Fig. 2). As the temperature changes, the design adapts so that the temperature-dependent current source charges in the same amount of time as the fixed current source.

This is accomplished via a digital feedback loop, which equalizes the charging times by connecting the temperature-dependent source to capacitors of different sizes, with the size of the capacitor directly proportional to the temperature being assessed. As the temperature falls, the temperature-dependent source will charge more slowly. Therefore, the feedback loop adjusts by switching in a smaller capacitor, which in turn points to a specific readout value.

3. Depicted are the experimental results of the intrinsic oscillator and temperature sensor: Measured oscillation frequency of the intrinsic oscillator across temperature at V_DD = 0.5 V (A). Measured power of the intrinsic oscillator across temperature at V_DD = 0.5 V (B). Measured temperature error across temperature at V_DD = 0.5 V (C). Measured power of the temperature sensor across temperature at V_DD = 0.5 V (D).

Accuracy was on the order of ±1°C (Fig. 3C), while power consumption was a little over 100 pW, rising with temperature (Fig. 3D). The complete, detailed technical paper, “Near-Zero-Power Temperature Sensing via Tunneling Currents Through Complementary Metal-Oxide-Semiconductor Transistors,” is available online at Nature Scientific Reports, complete with schematics, equations, operating principles, analysis, graphs, and performance results.