We study the rf-sputtered MgZnO/ZnO heterostructure material system. Two dimensional electron gases (2DEGs) are observed in the defective polycrystalline material system. As the MgZnO thickness increases, the sheet resistance reduces rapidly and then saturates. The enhancement of the interfacial polarization effect becomes stronger, corresponding to a larger amount of resistance reduction, when the Mg content in the MgZnO layer increases. Aldoped MgZnO is then introduced as the modulation doping layer to further raise the carrier density without diminishing the carrier mobility. Finally, rf-sputtered MgZnO/ZnO heterostructure thin film transistors (TFTs) with coplanar top-gate geometry are successfully demonstrated. The threshold voltage (Vth) field-effect mobility (mu(FE)) and on/off current ratio (ON/OFF) of the Mg0.2Zn0.8O/ZnO heterostructure thin film transistor are -0.55 V, 84.2 cm(2)V(-1) s(-1), and 2 x 10(6), respectively. In comparison to the ZnO counterpart (VTH- 0.47V, mu(FE)= 1.5 cm(2) V-1 s(-1), ON/OFF = 10(5)), the polarization effect truly increases the carrier concentration at the interface and improves the field-effect mobility. The result suggests that the rf-sputtered polycrystalline MgZnO/ZnO material system can be a promising candidate for the application of low-cost large-area high-mobility devices.