Method for forming a MOS transistor and structure thereof

A method for forming a MOS transistor comprising the steps of:

• forming a semiconductor film on a substrate;
• forming an insulating film on said semiconductor film;
• forming a conductive material on said insulating film;
• patterning said conductive material into an island;
• removing a portion of said insulating film by etching to leave a portion of said insulating film unremoved under island and thin a portion of said insulating film outside the unremoved portion of said insulating film;
• forming a gate electrode by reducing a width of said island by removing a portion of said island by etching;
• and
• introducing an impurity into said semiconductor film with said gate electrode as a mask wherein the amount of said removed portion of the gate insulating film is controlled so that the impurity concentration in said semiconductor film under the thinned portion of said insulating film is rendered larger than that in said semiconductor film under the removed portion of said island by said introducing step.

A method for forming a MOS transistor comprising the steps of:

• forming a semiconductor island on an insulating surface of a substrate;
• forming an insulating layer on said semiconductor island;
• forming a gate electrode on a part of said insulating layer;
• and
• introducing an impurity to a portion of said semiconductor island at an acceleration voltage through said insulating layer with said gate electrode as a mask in order to form source and drain regions in said semiconductor island and a channel region therebetween,
• wherein said insulating layer has a first portion located under said gate electrode and extending beyond said gate electrode, and a second portion located on said source and drain regions and having a thickness thinner than said first portion,
• wherein the thickness of said first and second portions and said acceleration voltage are selected in order that LDD regions are formed between said channel region and said source and drain regions.