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Scaling down of semiconductor devices requires high- k dielectric materials to continue lowering the operating voltage of field-effect transistors (FETs) and storing sufficient charge on a smaller area. Here, we investigate the dielectric properties of epitaxial BaHf 0.6 Ti 0.4 O 3 (BHTO), an alloy of perovskite oxide barium hafnate (BaHfO 3 ) and barium titanate (BaTiO 3 ). We found the dielectric constant, the breakdown field, and the leakage current to be 150, 5.0 megavolts per centimeter (MV cm -1 ), and 10 -4 amperes per square centimeter at 2 MV cm -1 , respectively. The results suggest that two-dimensional (2D) carrier density of more than n 2D = 10 14 per square centimeter (cm -2 ) could be modulated by the BHTO gate oxide. We demonstrate an n-type accumulation mode FET and direct suppression of more than n 2D = 10 14 cm -2 via an n-type depletion-mode FET. We attribute the large dielectric constant, high breakdown field, and low leakage current of BHTO to the nanometer scale stoichiometric modulation of hafnium and titanium.