We fabricated metal oxide semiconductor field effect transistor (MOSFETs) with 0.5-nm-thick Hf-silicate/HfO2 gate dielectrics on SiON base layers of different nitrogen concentration and studied the effects of the nitrogen concentration on their electrical properties. The gate dielectrics were deposited by atomic layer deposition (ALD) technology using Hf[N(CH3)(C2H5)](4) and SiH[N(CH3)(2)](3) precursors. O-3 was used as an oxidant. Rapid thermal annealing (RTA) in a NO ambient gave rise to a SiON layer with a nitrogen concentration of about 7 atom %. With a NH3 + NO ambient, the nitrogen concentration increased to 16%. Ultrathin films with equivalent oxide thickness (EOT) of about 1.0 nm could be fabricated using Hf-silicate/HfO2 gate stacks on base layers of SiON. The leakage current density of devices with SiON base layers was about 1 order of magnitude less than that with a SiO2 base layer, and that increased about 100 times for every 0.4-nm decrease of EOT. The flatband voltage (V-FB) and the threshold (V-th) with SiON base layers were slightly lower than those with a SiO2 base layer, which implies that positive charge is generated due to nitrogen at the interface. The effective mobility for devices with Hf-silicate/HfO2 gate stacks on SiON base layers was less than those with a SiO2 base layer due to the higher interface trap density (N-it). Moreover, increasing the nitrogen concentration in the base layer led to an increase in the interfacial trap density, thereby decreasing the effective mobility. (c) 2005 The Electrochemical Society. All rights reserved.