Semiconductor device

A semiconductor device, comprising:

• an isolation structure, formed in a substrate to define a first active region, a second active region, and a channel active region therebetween, which are separated by the isolation structure;
• a field implant region, formed below a portion of the isolation structure around the first, the second, and the channel active regions, wherein the channel active region includes two first sides defining a channel width, a distance from each first side to a second side of a neighboring field implant region is d1, and a shortest distance from a third side of each first active region or each second active region to an extension line of each second side of the field implant region is d2, R=d1/d2, where 0.15≦R≦0.85;
• a gate structure, covering the channel active region and extending over a portion of the isolation structure;
• and
• source/drain doped regions, formed in the first active region and the second active region, respectively.