$Si/Si_{1-x}Ge_x$ MODFET structures, incorporating linearly-graded buffer layers have been grown by GaS Source Molecular Beam Epitaxy. The growth temperature of the graded layers has not significantly changed the distribution of misfit dislocation. However, the surface undulation and surface defects were increased with increasing growth temperature. In $Si/Si_{1-x}Ge_x$ MODFET structures, the densities of misfit dislocations near the Si-active layers were considerably reduced in comparison with the region of graded layers. The electron mobility of $Si/Si_{1-x}Ge_x$ MODFET structure has increased with lowering the growth temperature.