Thin film semiconductor device and manufacturing method therefor

A method for manufacturing a thin film semiconductor device, the method comprising steps of:

• (a) forming a gate electrode above a substrate;
• (b) forming an oxide semiconductor layer at a position above the substrate and opposing the gate electrode;
• (c) performing a plasma treatment on the oxide semiconductor layer by using gas containing oxygen;
• (d) forming a first insulating layer on the oxide semiconductor layer;
• and
• (e) forming a source electrode and a drain electrode each connected to the oxide semiconductor layer,
• wherein, in the performing step (c) the plasma treatment is performed in particular conditions that a density of states DOS [eV−1cm−3] of oxygen defects in the oxide semiconductor layer satisfies the following relationship:
• DOS≦1.710×1017×(Ec−E)2−6.468×1017×(Ec−E)+6.113×1017
• provided that 2.0 eV≦Ec−E≦2.7 eV,
• where Ec [eV] is an energy level of a conduction band edge of the oxide semiconductor layer and E [eV] is a predetermined energy level of the oxide semiconductor layer.