A semiconductor device comprises a recessed trench in a substrate, a gate insulating layer including a first portion and a second portion, the first portion having a first thickness and covering lower portions of sidewalls of the recessed trench and a bottom surface of the recessed trench, and the second portion having a second thickness and covering upper portions of the sidewalls of the recessed trench, the second thickness being greater than the first thickness, a gate electrode filling the recessed trench, a first impurity region having a first concentration and disposed at opposing sides of the gate electrode, and a second impurity region having a second concentration greater than the first concentration and disposed on the first impurity region to correspond to the second portion of the gate insulating layer.