The increasing demand in electronic portability imposes low power consumption as a key metric to analog and digital circuit design. Tunnel FET (TFET) devices have been explored mostly in digital circuits, showing promising results for ultra-low power and energy efficient circuit applications. The TFET presents a low inverse sub-threshold slope (SS) that allows a low leakage energy consumption, desirable in many digital circuits, especially memories.In this book, the TFET is explored as an alternative technology also for ultra-low power and voltage conversion and management circuits, suitable for weak energy harvesting (EH) sources. The TFET distinct electrical characteristics under reverse bias conditions require changes in conventional circuit topologies. In this book, ultra-low input power conversion circuits based on TFETs are designed and analyzed, evaluating their performance as rectifiers, charge pumps and power management circuits (PMC) for RF and DC EH sources.