Electron transport in quantum well modulation &dgr; doped on either the normal or the inverted side has revealed the major cause of the long‐puzzling inferior transport characteristics of the inverted interface. For growth conditions optimized for best transport with normal‐side doping, we find migration of the Si dopant toward the inverted interface during growth to be the primary reason for the reduced inverted well mobility. This new understanding has allowed us to grow modulation‐doped inverted quantum wells of unprecedented quality having electron mobilities as high as 2.4×106cm2/V s at 4.2 K and 3.0×106cm2/V s at 1.0 K.