Provided is a method of manufacturing a semiconductor device. The method may include etching a first conductive type semiconductor substrate to form a first trench, forming a second trench extending from the first trench, diffusing impurities into inner walls of the second trench to form a second conductive type impurity region surrounding the second trench, forming a floating dielectric layer covering inner walls of the second trench and a floating electrode filling the second trench, and forming a gate dielectric layer covering inner walls of the first trench and a gate electrode filling the first trench.