This brief presents a bipolar junction transistor (BJT)-based CMOS temperature sensor without trimming. A current-mode readout scheme with dynamic current gain compensation is proposed to reduce the error caused by the low current gain β of the substrate BJT in nanometer CMOS technologies. Combining this readout scheme with techniques, such as chopping and dynamic element matching (DEM), the sensor achieves a high untrimmed accuracy for auto-calibration in thermal management applications. Fabricated in a standard digital 55-nm CMOS process, the sensor shows a measured inaccuracy within ±1.7 °C (3σ) from -40 °C to 125 °C without calibration. It occupies a die area of 0.0146 mm2 and has a power consumption of 37 μW with an adjustable resolution from 12 to 15 bit and a conversion time of 4.1-32.8 ms.