A drastic enhancement of electron mobility was observed in Zn polar ZnMgO/ZnO heterostructures (ZnMgO on ZnO) grown by radical source molecular beam epitaxy (MBE) due to the formation of a two-dimensional electron gas (2DEG). The electron mobility dramatically increased with increasing Mg composition for a ZnMgO layer and the electron mobility (mu similar to 250 cm(2)/Vs) at RT reached a value more than twice that of an undoped ZnO layer (mu similar to 100 cm(2)/Vs). Reflection high energy electron diffraction (RHEED) patterns taken during the growth of the ZnMgO layer remained streaky; X-ray diffraction measurements showed no evidence of phase separation for up 44% Mg composition. Temperature-dependent Hall measurements of ZnMgO/ZnO heterostructures also exhibited properties associated with well-defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value of 2750 cm(2)/Vs at 4 K. Zn polar "ZnMgO on ZnO" structures are easy to adapt to top-gate devices. These results open new possibilities for high electron mobility transistors (HEMTs) using ZnO-based materials. (c) 2006 Published by Elsevier B.V.