Method for manufacturing thin film transistor having microcrystalline semiconductor film

A method for manufacturing a thin film transistor comprising the steps of:

• forming a gate electrode over a substrate having an insulating surface;
• forming a gate insulating layer over the gate electrode;
• forming a semiconductor layer over the gate insulating layer in the presence of plasma;
• forming a buffer layer over the semiconductor layer, the buffer layer comprising an amorphous semiconductor;
• forming source and drain regions over the buffer layer, the source and drain regions including an impurity element;
• and
• forming source and drain electrodes over the source and drain regions, respectively,
• wherein the step of the formation of the semiconductor layer includes:
• forming a first region using a semiconductor source gas and a gas containing nitrogen so that the first region has an amorphous structure;
• and
• forming a second region over the first region using the semiconductor source gas in the absence of the gas containing nitrogen so that the second region has a crystal particle of a semiconductor.