This disclosure provides systems, methods and apparatus for a thin film transistor (TFT) device on a substrate. In one aspect, the TFT device includes a gate electrode, an oxide semiconductor layer, and a gate insulator between the gate electrode and the oxide semiconductor layer. The oxide semiconductor layer includes at least two metal oxides, with the two metal oxides having a varying concentration relative to one another between a lower surface and an upper surface of the oxide semiconductor layer. The TFT device also includes a source metal adjacent to a portion of the oxide semiconductor layer and a drain metal adjacent to another portion of the oxide semiconductor layer. The composition of the oxide semiconductor layer can be precisely controlled by a sequential deposition technique using atomic layer deposition (ALD).