A layer structure comprising substrate, a metal layer, a first amorphous silicon layer, an insulating layer, and a second amorphous silicon layer, and a method of crystallizing the second amorphous silicon layer by irradiating single pulse laser to the layer structure are provided. The method provides an effect of forming large grain of amorphous silicon as good as using dual pulse laser or higher just by using single pulse laser without additional optical system. A semiconductor device employing the layer structure maximizes an electron mobility.