A semiconductor device fabrication method includes forming a substrate having first and second regions therein, with different densities of active regions in the first and second regions. A cell trench is formed, which defines cell active regions in the first region, and a peripheral trench is formed, which defines peripheral active regions in the second region. A first insulating layer is formed in the cell trench and the peripheral trench. A mask is selectively formed, which covers the first insulating layer in the first region and exposes the first insulating layer in the second region. A second insulating layer is formed on the first insulating layer in the second region exposed by the mask, using a selective dielectric-on-dielectric deposition process. The first insulating layer is exposed in the first region by removing the mask. A third insulating layer is formed on the first insulating layer in the first region and on the second insulating layer in the second region.