Cryogenic Characterization of 28-nm FD-SOI Ring Oscillators With Energy Efficiency Optimization

Extensive electrical characterization of ring oscillators (ROs) made in high-κ metal gate 28-nm fully depleted silicon-on-insulator technology is presented for a set of temperatures between 296 and 4.3 K. First, delay per stage (τP), static current (ISTAT), and dynamic current (IDYN) are analyzed for the case of the increase of threshold voltage (VTH) observed at low temperature. Then, the same analysis is performed by compensating VTHto a constant, temperature-independent value through forward body biasing (FBB). Energy efficiency optimization is proposed for different supply voltages (VDD) in order to find an optimal operating point combining both high RO frequencies and low-power dissipation. We show that the EnergyDelay product can be significantly reduced at low temperature by applying an FBB voltage (VFBB). We demonstrate that outstanding performance of RO in terms of speed (τP= 37 ps) and static current (7nA/stage) can be achieved at 4.3 K with VDDreduced down to 0.325 V.