A method for forming a fin transistor includes forming a fin active region, depositing a thin layer doped with impurities over a semiconductor substrate, and forming a channel by diffusing the impurities into the fin active region of the fin transistor. In detail of the fin transistor formation, a fin active region is formed, and a patterned pad nitride layer is formed over the fin active region. A thin layer containing boron is deposited over the fin active region and isolation regions. Boron in the thin layer is diffused into the fin active region to form a channel.