The response time of SiGe/Si modulation-doped multiple-quantum-well modulators is investigated. A refractive index variation is achieved by the depletion of the free carriers initially present in the wells. Both the tunneling and thermionic emissions are taken into account to study the time needed for the free carriers to escape from and to be captured into the wells. Results are presented for an optimized three-Si0.8Ge0.2-quantum-well (QW) (10-nm-thick) device. Such a QW structure can intrinsically reach an operation frequency around 13 GHz.