Modulation-doped SiGe/Si/SiGe heterostructures were grown by molecular-beam epitaxy (MBE), and n-type field-effect transistors were fabricated. Electrons transferred to the strained-Si channel increased the electron density to up to 1 x 10(12) cm(-2) with increasing gale voltage, then excess electrons remained in the SiGe doping layer under a higher gate voltage. A device simulation explained this phenomenon well and suggested that device performance can be improved by increasing the Ge content in the SiGe layer to similar to 0.5.