This article describes a 128-Gb/s pulse amplitude-modulation 4-level (PAM-4) transmitter (TX) implemented in a 14-nm CMOS FinFET technology. Equalization is provided by a fully reconfigurable 3-tap baud-spaced feed-forward equalizer (FFE). The TX uses a segmented tailless current mode logic (CML) driver topology. The key architectural and circuit techniques include the thermometer-encoded driver slices, the clock phase selection circuits to perform segment reassignment to different FFE taps, and coarse-fine tuning of the FFE tap weights. The measured energy efficiencies for PAM-4 signaling are 1.33 pJ/b at 128 Gb/s with 1-Vppd output amplitude and 1.0 pJ/b at 112 Gb/s with 0.6-Vppd output amplitude. These results represent the highest data rate and best energy efficiencies reported to date.