We describe new routes to the growth of Ge and Sn-containing semiconductors on Si(100). For modest Sn concentrations GeSn alloys are expected to be direct-gap materials and this property can be exploited to develop band-to-band devices. The ternary GeSiSn system eliminates one of the major limitations of SiGe/Si by decoupling strain and bandgap. This may lead to new families of devices including quantum cascade lasers and high-efficiency solar cells based on hybrid group IV/III-V designs. The latest advances in low-temperature CVD of SiGe/Si, Ge/Si, GeSn/Si, GeSiSn/GeSn/Si and GeSiSn/Ge/Si materials are described and key developments leading to practical device fabrication are emphasized. This includes selective growth via novel epitaxy and practical doping protocols via designer molecular sources to achieve carrier concentrations n, p > 1019 cm-3 for which alloy scattering to the electron and hole mobilities is very small. As an example of a GeSn/GeSiSn prototype device the fabrication of a simple photoconductor at 1.55 μm is presented.