This disclosure provides systems, methods and apparatus for fabricating thin film transistor devices. In one aspect, a substrate having a source region, a drain region, and a channel region between the source region and the drain region is provided. The substrate also includes an oxide semiconductor layer, a first dielectric layer overlying the channel region, and a first metal layer on the dielectric layer. A second metal layer is formed on the oxide semiconductor layer overlying the source region and the drain region. The oxide semiconductor layer and the second metal layer are treated to form a heavily doped n-type oxide semiconductor in the oxide semiconductor layer overlying the source region and the drain region. An oxide in the second metal layer also can be formed.