Provided are semiconductor devices and fabricating methods thereof. The semiconductor device includes a field insulating layer formed in a substrate, an interlayer dielectric layer formed on the field insulating layer and including a trench exposing at least a portion of the field insulating layer, a deposition insulating layer formed in the trench to be disposed on the field insulating layer, a gate insulating layer formed the trench to be disposed on the deposition insulating layer, and a metal gate formed the trench on the gate insulating layer.