Si/SiGe heterostructures were grown by molecular-beam epitaxy. With atomic-hydrogen irradiation during the growth of the heterostructure, characteristics of modulation-doped field-effect transistors (MODFETs) were improved. A Hall effect measurement and a device simulation revealed that carrier density capable of being confined in the Si channel is limited in the MODFETs. Surface-Channel metal-oxide-semiconductor field-effect transistors (MOSFETs) with strained-Si channel on various buffer layers were fabricated. Effective electron mobility in strained-Si MOSFETs with a Sice compositionally graded buffer layer was 60% higher than that of an unstrained-Si MOSFET. The enhancement of mobility varied with the types of the buffer layers. This variation may be attributed to the amplitude of surface roughness of the strained-Si layer.