Methods of forming a semiconductor device include forming a trench mask pattern on a semiconductor substrate having active regions and device isolation regions. A thermal oxidation process is performed using the trench mask pattern as a diffusion mask to form a thermal oxide layer defining a convex upper surface of the active regions. The thermal oxide layer and the semiconductor substrate are etched using the trench mask pattern as an etch mask to form trenches defining convex upper surfaces of the active regions. The trench mask pattern is removed to expose the convex upper surfaces of the active regions. Gate patterns are formed extending over the active regions.