Thin film transistor substrate having bi-layer oxide semiconductor

A thin film transistor substrate for a display device, comprising:

• a substrate;
• a first oxide semiconductor layer on or over the substrate;
• a second oxide semiconductor layer on the first oxide semiconductor layer,
• wherein the first and second oxide semiconductor layers include indium, gallium and zinc and have different composition ratios in indium, gallium and zinc, the first oxide semiconductor layer has a higher resistivity than the second oxide semiconductor layer, and a threshold voltage of the display device changes less than 1.0 voltage when a channel length is reduced.

A thin film transistor substrate for a display device, comprising:

• a substrate;
• a first oxide semiconductor layer on the substrate;
• a second oxide semiconductor layer on the first oxide semiconductor layer, wherein the first and second oxide semiconductor layers include indium, gallium and zinc and have different composition ratios in indium, gallium and zinc, the first oxide semiconductor layer a lower resistivity than the second oxide semiconductor layer, and a threshold voltage of the display device changes less than 1.0 voltage when a channel length is reduced;
• a gate insulating layer on the second oxide semiconductor layer;
• a gate electrode on the gate insulating layer and vertically overlapping a middle portion of the second oxide semiconductor layer;
• an intermediate insulating layer on the gate electrode;
• and
• a source electrode and a drain electrode on the intermediate insulating layer, wherein the source electrode contacts a first portion of the second oxide semiconductor layer through a source contact hole, and the drain electrode contacts a second portion of the second oxide semiconductor layer through a drain contact hole.