A semiconductor device includes a memory string that includes a plurality of memory cells and is coupled between a source line and a bit line. A method for operating the semiconductor device may include: boosting a first channel region in a channel region of the memory string, wherein the channel region includes the first channel region at one side of the selected memory cell and a second channel region at the other side of the selected memory cell; applying a pre-program bias to a gate electrode of the selected memory cell, to inject electrons into a space region of the selected memory cell; and applying a program bias to the gate electrode.