We report a new structure formed by δ doping the barrier of an AlGaAs heterostructure. Our structure contains a pair of electronically coupled quantum systems that develop from the quantum size effect occurring in the barrier and at the heterointerface. Compared to conventional homogeneous doping, we find enhancements in interface density resulting in δ‐doped structures for all spacer thicknesses. A mobility of 1.9×106 cm2/V s is observed at a spacer layer thickness of 360 Å. Systematic dependences of interface mobility and density on spacer thickness are deduced from the quantum Hall effect and variable temperature Hall measurements.