A semiconductor device includes a power metal-oxide-semiconductor (MOS) transistor including a semiconductor substrate, an impurity region on the semiconductor substrate, the impurity region having a first conductivity, a drift region in the impurity region, the drift region having the first conductivity, a body region in the impurity region adjacent to the drift region, the body region having a second conductivity different from the first conductivity, a drain extension insulating layer on the drift region, a gate insulating layer and a gate electrode sequentially stacked across a portion of the body region and a portion of the drift region, a drain extension electrode on the drain extension insulating layer, a drain region contacting a side of the drift region opposite to the body region, the drain region having the first conductivity, and a source region in the body region, the source region having the second conductivity.