We report record-high electron mobilities obtained in the Si/SiGe alloy system via single-junction n-type modulation-doped Si/Si0.7Ge0.9 heterostructurcs grown by the ultra-high vacuum chemical vapor deposition technique. Peak electron mobilities as high as 1,800 cm2/Vs, 9,000 cm2/Vs and 19,000 cm2/Vs were measured at room temperature, 77K and 1.4K, respectively. These high mobilities resulted from excellent Si/SiGe interfacial properties by employing a compositional graded Si/SiGe superlattice prior to the growth of a thick S0.7Ge0.3 buffer, which brought about a dramatic reduction of the threading dislocation density in the active Si channel. Two thin phosphorous-doped layers were incorporated in the SiGe barrier and at its surface to supply electrons to the Si channel and to suppress the surface depletion, respectively. The transport properties of these heterostructurcs were determined to be those of a two dimensional electron gas at Si/SiGe heterointerfaces at low temperatures.