Semiconductor device and method for fabricating the same

A method for fabricating a semiconductor device, comprising:

• providing a semiconductor layer, having dopants of a first conductivity type;
• forming a first doped well region and a second doped well region in a portion of the semiconductor layer, wherein the first doped well region has dopants of the first conductivity type, and the second doped well region has dopants of a second conductivity type opposite to the first conductivity type and a symmetric cross-sectional profile;
• forming first, second and third isolations over the semiconductor layer, wherein the first and second isolations partially cover a portion of the first doped well region and defines a first region between the first and second isolation, and the third isolation is adjacent to the second doped well region and defines a second region between the second and third isolations;
• forming a patterned mask layer having an opening therein over the semiconductor layer, wherein the opening exposes a portion of the second doped well region;
• forming a trench through the portion of the second doped well region exposed by the opening and a first doped region in a portion of the second doped well region exposed by the trench and a portion of the semiconductor layer under the second doped well region, wherein the first doped region has dopants of the first conductivity type;
• removing the patterned mask layer;
• performing a thermal diffusion process, diffusing the dopants of the first conductivity type of the first doped region into the second doped well region adjacent thereto, making the symmetric cross-sectional profile into an asymmetric cross-sectional profile, wherein a bottom surface of a portion of the second doped well region adjacent to the trench is closer to a top surface of the semiconductor layer than other portions of the second doped well region;
• forming a first gate structure over a portion of the semiconductor layer in the second region and a second gate structure in the trench, wherein the first gate structure partially covers the second isolation and the second doped well region;
• and
• forming second, third, fourth and fifth doped regions, wherein the second doped region is formed in a portion of the first doped well region and has dopants of the second conductivity type, and the third and fifth doped regions are formed in a portion of the second doped well region and has dopants of the first conductivity type, and the fourth doped region is formed in a portion of the second doped well region and is between the third and fifth doped regions and has dopants of the second conductivity type.