  Silicon photonics has emerged as the leading candidate for implementing
ultralow power wavelength division multiplexed communication networks in
high-performance computers, yet current components (lasers, modulators,
filters, and detectors) consume too much power for the femtojouleclass links
that will ultimately be required. Here, we propose, demonstrate, and
characterize the first modulator to achieve simultaneous high-speed (25-Gb/s),
low voltage (0.5VPP) and efficient 1-fJ/bit error-free operation while
maintaining athermal operation. Both the low energy and athermal operation were
enabled by a record free-carrier accumulation/depletion response obtained in a
vertical p-n junction device that at 250-pm/V (30-GHz/V) is up to ten times
larger than prior demonstrations. Over a 7.5{\deg}C temperature range, the
massive electro-optic response was used to compensate for thermal drift without
increasing energy consumption and over a 10{\deg}C temperature range,
increasing energy consumption by only 2-fJ/bit. The results represent a new
paradigm in modulator development, one where thermal compensation is achieved
electro-optically.
