Semiconductor device and manufacturing method thereof

A method comprising:

• forming an epitaxial layer over a semiconductor substrate;
• forming a first well region over said epitaxial layer;
• forming a first isolation layer and a third isolation layer at opposite sides of said first well region;
• forming a second well region over said first well region between said third isolation layer and said second isolation layer;
• forming a first ion-implanted region over said second well region;
• forming a second ion-implanted region between said first ion-implanted region and said second isolation layer;
• forming a third ion-implanted region between said second isolation region and said first isolation region;
• diffusing said second well region using a thermal treatment over a region between said second well region where said second ion-implanted region is not formed and a region where a gate is to be formed;
• and
• forming an accumulation channel over said diffused second well region to said region where said gate is to be formed.