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

Extensive electrical characterization of ring oscillators (ROs) made in high-kappa 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 (tau(P)), static current (I-STAT), and dynamic current (I-DYN) are analyzed for the case of the increase of threshold voltage (V-TH) observed at low temperature. Then, the same analysis is performed by compensating V-TH to a constant, temperature-independent value through forward body biasing (FBB). Energy efficiency optimization is proposed for different supply voltages (V-DD) in order to find an optimal operating point combining both high RO frequencies and low-power dissipation. We show that the Energy-Delay product can be significantly reduced at low temperature by applying an FBB voltage (V-FBB). We demonstrate that outstanding performance of RO in terms of speed (tau(P) = 37 ps) and static current (7nA/stage) can be achieved at 4.3 K with V-DD reduced down to 0.325 V.