Transistor fabrication method

A transistor fabrication method comprising the steps of:

• sequentially forming a pad oxide film and a silicon nitride film on a semiconductor substrate while exposing at least a portion thereof;
• etching the substrate to form a trench, using the pad oxide film and the silicon nitride film as a mask;
• sequentially forming a first oxide film within the trench and a cylindrical insulation spacer at a lateral portion of the first oxide film;
• forming an insulation pattern for covering the inside of the trench, the first insulation film and the insulation spacer;
• etching the silicon nitride film, the insulation pattern and the insulation spacer to expose the pad oxide film;
• removing the pad oxide film;
• removing the insulation spacer and the first oxide film;
• sequentially forming source/drain regions and LDD regions on the underlying substrate at both sides of the trench, under the remaining insulation pattern;
• forming a second oxide film on the substrate including the source and drain regions;
• sequentially forming a channel stop layer between the LDD regions and a punch stop layer under the channel stop layer;
• and
• sequentially forming a gate insulation film and a gate region within the trench and the second oxide layer.