A transistor includes a gate structure over a substrate, wherein the substrate includes a channel region. The transistor further includes a source/drain (S/D) in the substrate adjacent to the gate structure. The transistor further includes a lightly doped drain (LDD) region adjacent to the S/D, wherein a dopant concentration in the first LDD is less than a dopant concentration in the S/D. The transistor further includes a doping extension region adjacent the LDD region, wherein the doping extension region extends farther under the gate structure than the LDD region, and a maximum depth of the doping extension region is 10-times to 30-times greater than a maximum depth of the LDD.