A method of fabricating a semiconductor device includes first providing an insulation substrate. A patterned conductive layer is formed over the insulation substrate, and the patterned conductive layer includes a channel region and a number of protruding regions. A gate structure layer is formed over the insulation substrate. The gate structure layer covers a part of the patterned conductive layer, and each of the protruding regions has an exposed region. A doping process is performed to dope at least the exposed region of the patterned conductive layer to form a number of S/D regions.