1. A method of fabricating a semiconductor device, comprising: forming a conductive layer on a semiconductor substrate; forming an insulating layer on the conductive layer; forming a word line and isolation trenches by patterning the insulating layer and the conductive layer; forming an isolation layer that fills the isolation trenches; forming a cell contact hole in the insulating layer such that the cell contact hole is self-aligned with the word line and exposes the word line; and forming a cell diode in the cell contact hole.