Method for fabricating transistor

A method for fabricating a transistor, comprising:

• providing a substrate having a sacrificial layer, a metal layer and an insulating layer in turn formed thereon;
• forming a patterned masking layer over the insulating layer for defining a first gate window;
• removing a portion of the insulating layer and the metal layer using the patterned masking layer as a mask to formed the first gate window, and a portion of the sacrificial layer is exposed to the first gate window;
• laterally removing a portion of the patterned masking layer;
• laterally removing a portion of the remained insulating layer using the remained masking as a mask to form a second gate window, wherein the width of second gate window is greater than the width of the first gate window, and the first and the second gate windows form a T-shape gate window;
• completely removing the patterned masking layer;
• performing a thermal process to form a source and a drain regions by a reaction between the metal layer and the sacrificial layer under the metal layer;
• removing the sacrificial layer within the T-shape gate window;
• forming a gate dielectric layer over the remained insulating layer and the source and drain regions;
• and
• forming a gate electrode within the T-shape gate window.