A detailed synthesis study has been performed on a functional unit from a recent IBM microprocessor to explore the voltage-frequency space for energy-efficient design points across a wide performance spectrum ranging from 625 MHz at 0.48V to 5.6 GHz at 0.95V. It is found that the optimal operating voltage depends strongly on frequency for an energy-efficient design. Circuit characteristics, as represented by the combination of the average gate width, effective VT, and buffering scheme, differ significantly between designs optimized for low voltage-frequency and for high voltage-frequency operations and suggest a distinct application dependence in the selection of standard cell images and optimal design points. In particular, for optimal energy efficiency at a given frequency, low voltage designs should utilize smaller gate width and lower VT. Though a design energy-optimized near 1V is more scalable over a wide frequency range when operating at low voltages, designs optimized at a lower voltage-frequency point can be leveraged to offer better solutions in both performance and energy efficiency within a narrow frequency range near the design point.