The electroabsorption modulators (EAM) play an important role in the optoelectronic integrated circuits. InGaAs/InAlAs is an excellent material system for fabricating 1.55-£gm EA modulators. Natural high band-offset ( ) ratio structure and the strong exciton effect make this kind of material a good candidate for high saturation power operation and high speed. In order to design good quantum-well (QW) for electroabsorption, the Vegard¡¦s law is used to obtain the parameters of In1-x-yGaxAlyAs material by interpolating the relevant binary semiconductor material. The bowling factor is included in finding the right bandgap. Using the Resonant Scattering Method, the QW energy levels and electron-hole overlap integrals can be obtained to calculate the optical electroabsorption effects. In this thesis, the TWEAM based on In1-x-yGaxAlyAs material is also fabricated. In order to get low parasitic capacitance for high-speed operation, a processing called undercut-etching the active region is developed in this work. A selective etching solution (citric acid : H2O2) is used to etch InGaAs layers from InAlAs and the undercut-etching structure InGaAlAs EAM has been successfully fabricated. The processing includes 1)optical waveguide formation with wet etching; 2)n-contact evaporation and contact annealing; 3)mesa etch, PMGI passivation/bridging/planarization; 4)final metallization, cleaving line formation, wafer lapping and device cleaving.