New designs for the velocity matching of a deep-etched semiconductor electro-optic modulator are presented. A tantalum pentoxide (Ta2O5) coating is considered here for achieving velocity matching between the microwave and the optical signals. The effects of the velocity mismatch, the conductor loss, the dielectric loss, and the impedance mismatch are studied in relation to the optical bandwidth of a high-speed semiconductor modulator. It is shown that both the dielectric loss and the impedance matching play key roles for velocity-matched high-speed modulators with low conductor loss. The effects of Ta2O5 thickness on the overall bandwidth and on the half-wave voltage-length product VpiL are also reported.