High voltage semiconductor devices and methods for fabricating the same

A semiconductor device capable of high-voltage operation, comprising:

• a substrate comprising a first well formed therein;
• a gate stack overlying the substrate, wherein the gate stack comprises a gate dielectric layer and a gate electrode formed thereon;
• a channel well and a second well formed in portions of the first well, located on opposite sides of the gate stack;
• a source region formed in a portion of the channel well;
• and
• a drain region formed in a portion of the second well, wherein the first well and the second well is formed with a conductivity type opposite to that of the substrate, the channel well is formed with a conductivity type the same as that of the substrate;
• wherein the gate dielectric layer comprises a relatively thinner portion at one end of the gate stack adjacent to the source region and a relatively thicker portion at one end of the gate stack adjacent to and directly contacts the drain region.