30-mV Supply Ring Oscillator Employing Recursive-Stacking Body-Bias Inverters for Extremely-Low-Voltage Energy Harvesting

This paper presents ring oscillators (ROSCs) capable of operating at extremely low supply voltages. For inverters to have extremely low-voltage operation, we developed dedicated low-voltage inverters: a recursive stacking inverter (RS-INV) and a recursive stacking body-bias inverter (RS-BBI). The RS-INV is based on a conventional stacked inverter and consists of additional inverters stacked on the top and bottom of the inverters recursively. To further enhance low-voltage operation, we propose an RS-BBI that combines the RS-INV with the body-bias technique. Measurement results indicated that our ROSCs using RS-BBIs were able to operate at a lower supply voltage VDD as the number of stacking inverters increased. The measured lowest VDD,min was 26 mV even though not all chips were able to oscillate. All chips oscillated successfully at 30 mV.