In operation 230, two LDD features 118 are formed in the middle portion 106A of the active region 106, as shown in FIG. 3C. Each LDD feature 118 is disposed at an edge portion (114E or 114F) of the gate dielectric layer 116 and separates it from the isolation features 104. Profiles of the LDD features 118 on two sides of the gate stack 114 may or may not be symmetrical. In some embodiments, only one side of the gate stack 114 has the LDD feature. The LDD feature 118 may be formed using any suitable technique, for example, by ion implantation. The LDD feature 118 has the same dopant type as the channel region 124 but has a doping concentration at least twice as high as that of the channel region 124. In some examples, the channel region 124 may have a doping concentration between 1-5*E13 (unit is per square centimeter), while the LDD feature 118 may have a doping concentration between 6-9*E13. In some embodiments, to ensure full doping of the LDD feature 118 with desired doping concentration in the desired region, from the top view perspective the doping area may be designed to be relatively large (since doping is shallower on the edge of the doping area).