Method of forming insulating layer and method of fabricating thin film transistor using the same

A method of fabricating a thin film transistor, comprising the steps of:

• forming an active layer on a substrate;
• forming a first insulating layer on the active layer;
• forming a gate electrode on the first insulating layer to be isolated from the active layer;
• forming source/drain regions by implanting ions in portions of the active layer at both sides of the gate electrode, respectively;
• forming a second insulating layer on the first insulating layer such that density of the second insulating layer gradually changes from a bottom surface of the second insulating layer to a top surface of the second insulating layer, wherein density of the top surface of the second insulating layer is larger than density of the bottom surface of the second insulating layer;
• forming contact holes in the first and second insulating layers that expose the source/drain regions, respectively;
• and
• forming source/drain electrodes connected to the source/drain regions through the contact holes, respectively.