Transistor structures for a transistor may include a first source-drain region, a second source-drain region, and a channel region between the first and second source-drain regions overlapped by a gate structure. Transistor structures may be formed in a well of a first doping type. Isolation structures having the first doping type may be formed within the well. A lightly doped implant region of a second doping type for each of the source-drain regions may be formed within the well and separated from the isolation structures. A heavily doped surface implant region of the first doping type may extend across the surface of the well and cover the lightly doped implant region of each source-drain region. The surface implant region may be formed by patterning or by a blanket implantation process across the transistor structures.