The advantage of using the Al2O3 gate dielectric is discussed from thermodynamic and kinetic standpoints. We fabricated n-field effect transistors with Al2O3 dielectrics and built-in interfacial SiO2 having thicknesses of 0.6-2.0 nm to investigate the effect on electron mobility and gate leakage current. The mobility was reduced and the shift in flat-band voltage was increased with decreases of the thickness of the interfacial SiO2. We propose that a fixed charge is generated at the interface of Al2O3/ interfacial SiO2, it induced Coulomb scattering to the surface channel, and that the electron mobility is consequently reduced. The plot of gate leakage current against the total physical thickness, which includes Al2O3 and interfacial SiO2 dielectrics, showed the enhanced leakage current. This suggests that the band offset may be lowered at the interface of ultra-thin SiO2/Si beneath the Al2O3 dielectric. (C) 2003 Elsevier Science B.V. All rights reserved.