Method for forming a MOS transistor having lightly dopped drain regions and structure thereof

An active matrix display device including a thin film transistor for switching a pixel of said display device, said active matrix display device comprising:

• a substrate having an insulating surface;
• a semiconductor layer formed on said insulating surface, said semiconductor layer including at least a channel region and source and drain regions;
• an insulating film including a gate insulator therein formed on said semiconductor layer and having a thicker portion and thinner portions, wherein said source and drain regions are covered by the thinner portions of the insulating film and introduced with a dopant impurity through said insulating film at a first concentration, and wherein said channel region is covered by the thicker portion of said insulating film, wherein said source and drain regions extend throughout the thickness of the semiconductor layer;
• first and second LDD regions formed between the channel region and the source region and between the channel region and the drain region, wherein said first and second LDD regions are introduced with said dopant impurity through said insulating film, wherein a width of said first LDD region is substantially the same as a width of said second LDD region;
• a gate electrode formed on said thicker portion of the insulating film;
• an interlayer insulating film formed on said gate electrode and said insulating film;
• at least one electrode formed over said interlayer insulating film for contacting at least one of said source and drain regions,
• wherein said extending parts of the thicker portion extend beyond edges of said channel region and cover said LDD regions,
• wherein extending parts of the thicker portion and said thinner portions have first surfaces being in contact with said semiconductor layer and second surfaces being in contact with said interlayer insulating film,
• wherein said at least one electrode extends through said interlayer insulating film and said thinner portion of said insulating film, and
• wherein a concentration of the dopant impurity contained in said source and drain regions monotonically decreases in a thickness direction of the semiconductor layer at least partly toward said insulating surface.

An active matrix display device including a thin film transistor, said active matrix display device comprising:

• a substrate having an insulating surface;
• a semiconductor layer formed on said insulating surface;
• an insulating film including a gate insulator therein formed on said semiconductor layer, said insulating film including:
• a thicker portion located on and completely covering a channel region of said semiconductor layer, and
• thinner portions located on source and drain regions of said semiconductor layer, wherein said source and drain regions extend throughout a thickness of the semiconductor layer;
• a gate electrode formed on the thicker portion of said insulating film;
• first and second LDD regions formed in said semiconductor layer between the channel region and the source region and between the channel region and the drain region below extending parts of the thicker portion of said insulating film, said extending parts of the thicker portion extending beyond edges of the gate electrode, wherein a width of said first LDD region is substantially the same as a width of said second LDD region;
• an interlayer insulating film formed on said insulating film and said gate electrode;
• and
• at least one electrode formed over said interlayer insulating film for contacting at least one of said source and drain regions,
• wherein said extending parts of the thicker portion and said thinner portions have first surfaces being in contact with said semiconductor layer and second surfaces being in contact with said interlayer insulating film,
• wherein said at least one electrode extends through said interlayer insulating film and said thinner portion of said insulating film, and
• wherein a concentration of an impurity contained in said source and drain regions monotonically decreases in a thickness direction of the semiconductor layer at least partly toward said insulating surface.