A transistor includes an isolation region surrounding an active region. The transistor also includes a gate dielectric layer over a portion of the active region. The transistor further includes a gate electrode over the gate dielectric layer. The portion of the active region under the gate dielectric layer includes a channel region between a drain region and a source region, and at least one wing region adjoining the channel region. The at least one wing region has a base edge adjoining the channel region. The at least one wing region is polygonal or curved.