At a lattice-matched ZnO/ScAIMgO(4) heterointerface, Hall and field-effect mobilities of grain-boundary-free ZnO channels have been simultaneously characterized under a gate electric field (E-G) applied through a ScAIMGO(4) dielectric gate. The field-effect mobility increased linearly with increasing EG (see Figure), clearly in contrast to the supralinear (exponential) dependence that has been previously reported for polycrystalline channels.