A semiconductor device, includes an insulating film formed on a substrate; a conductive layer, comprising first and second doped poly-silicon regions and a undoped poly-Si region, formed on the insulating film; a highly doped first conductivity type drain region and a highly doped a first conductivity type source region formed in the first and second doped poly-silicon regions, respectively; and a highly doped second conductivity type gate region formed in the undoped poly-Si region between the highly doped first conductivity type drain region and the highly doped first conductivity type source region. The undoped poly-Si region is disposed closer to the highly doped first conductivity type source region than the highly doped first conductivity type drain region.