A method of fabricating a semiconductor device includes: forming a first polysilicon layer having a first thickness in a peripheral circuit region formed on a substrate; forming a stack structure comprising a first tunneling insulating layer, a charge trap layer, and a blocking insulating layer in a memory cell region formed on the substrate; forming a second polysilicon layer having a second thickness that is less than the first thickness on the blocking insulating layer; and forming gate electrodes by siliciding the first and second polysilicon layers.