The aim of this study, to explain effects of the SiO2 insulator layer thickness on the electrical properties of Au/n-GaAs Shottky barrier diodes (SBDs). Thin (60 ) and thick (250 ) SiO2 insulator layers were deposited on n-type GaAs substrates using the plasma enganced chemical vapour deposition technique. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics have been carried out at room temperature. The main electrical parameters, such as ideality factor (n), zero-bias barrier height (I center dot (Bo) ), series resistance (R (s) ), leakage current, and interface states (N (ss) ) for Au/SiO2/n-GaAs SBDs have been investigated. Surface morphologies of the SiO2 dielectric layer was analyzed using atomic force microscopy. The results show that SiO2 insulator layer thickness very affects the main electrical parameters. Au/n-GaAs SBDs with thick SiO2 insulator layer have low leakage current level, small ideality factor, and low interface states. Thus, Au/n-GaAs SBDs with thick SiO2 insulator layer shows better diode characteristics than other.