An embodiment for realizing a power device with trench-gate structure integrated on a semiconductor substrate, and including etching the semiconductor substrate to make a first trench having first side walls and a first bottom; and further etching said semiconductor substrate to make a second trench inside the first trench, realized in a self-aligned way and below this first trench, the first trench and the second trench defining the trench-gate structure with a bird beak-like transition profile suitable for containing a gate region.