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 (τ
_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 EnergyDelay 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 (τ
_P
= 37 ps) and static current (7nA/stage) can be achieved at 4.3 K with V
_DD
reduced down to 0.325 V.