Abstract:A new design of Mach Zehnder traveling wave interferometer modulator based on the slow wave effect of the n +heavy doped epitaxial layer are proposed the first time in this paper.When applying the method of line to finite conductivity structure,the permittivity of semiconductor is assumed to be complex.The influences on the microwave characteristics of the structural parameters of the modulator are analyzed in details by means of the method of line.Based on the analytical results,we get the structural parameters which is needed so as to obtain velocity match and the minimum loss.Its 3dB bandwidth is estimated around 30 GHz.We also propose a new optimal grooved structure,which can reduce the gap of electrodes and the thickness of waveguide,so as to make the modulator's 3 dB bandwidth to be expanded largely.