In this paper, we investigate that the spacer layer thickness affects the polarization effect on the two-dimensional electron gas (2DEG) concentration and current-voltage properties in an MgZnO/ZnO hetero structure. In the conduction band edge profile, MgZnO/ZnO hetero structure with a thick layer of spacer shows sharper bending. As a result, the MgZnO/ZnO hetero structure with a relatively thick layer of spacer is expected to surpass the MgZnO/ZnO hetero structure with a thin layer of spacer in the carrier confinement. However, if the spacer layer thickness exceeds 30nm, the carrier concentration will no longer increase. The drain current and pinch-off voltage have been shown to be proportional to the spacer layer thickness from the addition in electron density due to the increased spacer layer thickness.