In advanced high-speed integrated systems, the widely distributed and proliferation of temperature sensors to detect hotspots improve the robustness and reliability of the system by preventing overheating. Low area and low energy consumption are essential for integrated temperature sensors in such applications. The band-to-band tunneling (BTBT) current in PDSOI technology increases linearly with temperature. We demonstrate a BTBT-based quasi-linear temperature-dependent oscillator, thermillator. The fabricated thermillator has a low footprint (42.3 \(\mu m^{2}\), 10\(\times\) less than state-of-the-art temperature sensing cores) and enables low energy consuming temperature to digital converter (0.33 nJ energy/conversion) in GF45RFSOI technology. The proposed thermillator facilitates a low power and low footprint temperature sensor (0 \({}^{o}C\) to 85 \({}^{o}C\)) with a simple counter-based digital readout with a best-in-class resolution FoM of 0.16 \(pJK^{2}\).