A method for forming a semiconductor device structure is provided. The method includes forming an n-type doped region in a semiconductor substrate and forming a semiconductor stack over the semiconductor substrate. The semiconductor stack has multiple sacrificial layers and multiple semiconductor layers laid out alternately. The method also includes introducing n-type dopants from the n-type doped region into the semiconductor stack during the forming of the semiconductor stack. The method further includes patterning the semiconductor stack to form a fin structure and forming a dummy gate stack to wrap around a portion of the fin structure. In addition, the method includes removing the dummy gate stack and the sacrificial layers to release multiple semiconductor nanostructures made up of remaining portions of the semiconductor layers. The method includes forming a metal gate stack to wrap around the semiconductor nanostructures.