In this thesis, a low-power, high-accuracy BJT-based temperature sensor is proposed, which consists of two main parts: a BJT-based dual-mode front-end (DMFE) and a tracking Delta-Sigma-Modulator (DSM). The front-end employs vertical PNP transistors, which, compared to NPNs, exhibit fewer non-idealities when biased at nA-level currents. The proposed DMFE generates accurate proportional-to-absolute-temperature (PTAT) and complementary-to-absolute-temperature (CTAT) voltages, while only consuming half the power of a conventional front-end. The temperature dependent ratio of these voltages is then digitized by an energy-efficient tracking DSM. The prototype sensor was fabricated in a 0.18µm CMOS process and has an active area of 0.228mm2. Measurement results show that the sensor achieves an inaccuracy of ±0.15°C (3σ) over the industrial temperature range (-45°C to 85°C) after 1-point temperature calibration, which corresponds to a relative inaccuracy of 0.3%. It also achieves a 32mK resolution in 100ms, while consuming only 130nW. Its low power consumption, accuracy, and resolution make it suitable for IoT applications.