The electrical and structural properties of HfO2/SrTiO3 composite gate dielectric are discussed for achieving higher dielectric constant to reduce equivalent oxide thickness (EOT) as well as to tune threshold voltage (V-TH) in n-type metal oxide semiconductor transistor. Compared to atomic layer deposited (ALD) HfO2 alone, adding 0.5-1.0 nm sputtered SrTiO3 layer into ALD HfO2 gate dielectric attains EOT scaling down to about 0.6 nm, similar to 700 mVnegative V-TH shift, comparable gate leakage current at both fixed 1 V and V-TH + 0.8 V overdrive conditions and electron mobility with of 137 cm(2)/V.s at 0.6 nm EOT. This is attributed to the significant modification of the interfacial layer (IL), resulting from Sr diffusion into sub-SiOx IL during subsequent anneal in gate-first transistor processes. It leads to SrOx IL formation between high-k gate insulator layer and Si substrate, where its dielectric constant is about three times higher than that of SiOx IL. (C) 2012 Elsevier B.V. All rights reserved.