SiGe quantum well structures gain increasing interest in the Si technology. The preparation of a Si channel or a Ge-rich or even a pure cc channel with a respective two-dimensional carrier gas opens the attractive possibility to fabricate high performance n- or p-type field effect transistors. For both device types, a virtual substrate surface is required which is created by a strain relieved buffer layer grown on a Si standard wafer. The paper reviews various approaches of SiGe buffers including special attempts to reduce the thickness and to improve the quality. N- and p-type modulation-doped field-effect transistors are presented which show comparably good device characteristics and cut-off frequencies in the range of 100-120 GHz.