This study reports the dielectric and leakage current properties of Mg2Hf5O12 thin films deposited at room temperature by radio-frequency magnetron sputtering. Polycrystalline Mg2Hf5O12 thin films showed a reasonably high dielectric constant (epsilon(r) = 22) and greatly enhanced leakage current characteristics (<2 x 10(-7) A/cm(2)) compared to the leakage current (similar to 2 x 10(-5) A/cm(2)) of HfO2 thin films at 0.4 MV/cm. A bandedge spectroscopic analysis revealed lower conduction bandedge defect states and a greater p-type-like Fermi energy level of Mg2Hf5O12 compared to the HfO2 thin films. The suitability of Mg2Hf5O12 films as gate insulators for low voltage operating InGaZnO4 thin film transistors (TFTs) was investigated. All room-temperature processed InGaZnO4 TFTs on plastic substrates exhibited a high field effect mobility of 27.32 cm(2)/Vs and a current on/off ratio of 4.01 x 10(6). The threshold voltage and subthreshold swing were 2 V and 440 mV/dec, respectively. The fabrication and compositional manipulation method of the Mg2Hf5O12 films described in this work is simple and versatile, providing fascinating opportunities for new high-k gate dielectrics. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3465656] All rights reserved.