Abstract-Quantum-confined Stark effect (QCSE) of group IV Ge(Sn)/SiGe(Sn) quantum wells (QWs) on Si substrate is analyzed by microscopic many-body theory for mid-infrared Si-based electroabsorption devices. To show the validity of the theory, QCSE of Ge/SiGe QW is investigated and very good agreement between theory and reported measured results is obtained. Next, QCSE of GeSn/SiGeSn QWs is analyzed and the QW design for electroabsorption modulators to obtain large extinction ratio in mid-infrared region is presented. It is shown that compressive and tensile strained well and barrier layers is preferable to obtain large extinction ratio due to its large conduction band offset.Index Terms -Si photonics, GeSn quantum wells, electroabsorption modulators, quantum-confined Stark effect, and many-body theory.