A synchronous-traveling carrier photodetector (STC-PD) with a thick absorption region and large diameter is presented and investigated for 100 Gbps PAM4 applications. In the STC structure, a suitable electric field is introduced to synchronize the traveling of electrons and holes, which achieves high bandwidth while maintaining large responsivity. The characteristics of STC-PD, including electric field distribution, energy band diagram, responsivity, junction capacitance, and frequency response, have been studied carefully.