We present an equivalent circuit model for a silicon carrier-depletion single-drive push–pull Mach–Zehnder modulator (MZM) with its traveling wave electrode made of coplanar strip lines. In particular, the partialcapacitance technique and conformal mapping are used to derive the capacitance associated with each layer. The PN junction is accurately modeled with the fringe capacitances taken into consideration. The circuit model is validated by comparing the calculations with the simulation results. Using this model, we analyze the effect of several key parameters on the modulator performance to optimize the design. Experimental results of MZMs confirm the theoretical analysis. A 56 Gb/s on–off keying modulation and a 40 Gb/s binary phase-shift keying modulation are achieved using the optimized modulator.