A semiconductor device and a fabrication method thereof are provided. The semiconductor device includes a P type well region and an N type well region formed in a substrate, a gate insulating layer having a non-uniform thickness and formed on the P type well region and the N type well region, a gate electrode formed on the gate insulating layer, a P type well pick-up region formed in the P type well region, and a field relief oxide layer formed in the N type well region between the gate electrode and the drain region.