Method of forming a MOS transistor in a semiconductor device and a MOS transistor fabricated thereby

A method of forming a MOS transistor, comprising:

• providing a semiconductor substrate of a first conductivity type;
• forming a second conductivity type well at a predetermined region of the semiconductor substrate of the first conductivity type;
• forming a first insulated gate pattern having sidewalls and a second insulated gate pattern having sidewalls on a predetermined region of the semiconductor substrate of the first conductivity type, and on a predetermined region of the well, respectively so that portions of the semiconductor substrate of the first conductivity type on both sides of the first insulated gate pattern remain exposed, and portions of the well on both sides of the second insulated gate pattern remain exposed;
• forming second conductivity type impurity regions on portions of the semiconductor substrate of the first conductivity type at both sides of the first insulated gate pattern that remain exposed;
• forming first conductivity type impurity regions on portions of the well at both sides of the second insulated gate pattern that remain exposed;
• forming a first interlayer insulating layer on the entire surface of the resultant structure having the first and second conductivity type impurity regions and the first and second insulated gate patterns;
• patterning the first interlayer insulating layer to form pad contact holes, thereby exposing the first and second conductivity type impurity regions so that portions of the first interlayer insulating layer remain disposed on the first and second insulated gate patterns;
• forming a conformal undoped buffer layer on the entire surface of the resultant structure having the pad contact holes and the portions of the first interlayer insulating layer;
• ion-implanting second conductivity type impurities in the conformal undoped buffer layer that is disposed on the second conductivity type impurity regions to form a second conductivity type buffer layer;
• ion-implanting first conductivity type impurities in the conformal undoped buffer layer that is disposed on the first conductivity type impurity regions to form a first conductivity type buffer layer;
• forming sacrificial insulating layer residues covering the first and second conductivity type buffer layers on bottoms of the pad contact holes;
• and
• etching the first and second conductivity type buffer layer using the sacrificial insulating layer residues as etching masks to form a first conductivity type pad and a second conductivity type pad on the first conductivity type impurity region and on the second conductivity type impurity region, respectively.