The present disclosure describes structure with a substrate, a first well region, a second well region, and a third well region. The first well region is in the substrate. The second well region is in the first well region and includes a first source/drain (S/D) region. The third well region is in the substrate and adjacent to the first well region. The third well region includes a second S/D region, where a spacing between the first and second S/D regions is less than about 3 μm.