  Extensive electrical characterization of ring oscillators (ROs) made in
high-$\kappa$ metal gate 28nm Fully-Depleted Silicon-on- Insulator (FD-SOI)
technology is presented for a set of temperatures between 296 and 4.3K. 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 ($EDP$) can be significantly reduced at low
temperature by applying a forward body bias voltage ($V_{FBB}$). We demonstrate
that outstanding performance of RO in terms of speed ($\tau_P$=37ps) and static
power (7nA/stage) can be achieved at 4.3K with $V_{DD}$ reduced down to 0.325V.
