A semiconductor device and method for forming the same provide a through silicon via (TSV) surrounded by a dielectric liner. The TSV and dielectric liner are surrounded by a well region formed by thermal diffusion. The well region includes a dopant impurity type opposite the dopant impurity type of the substrate. The well region may be a double-diffused well with an inner portion formed of a first material and with a first concentration and an outer portion formed of a second material with a second concentration. The surrounding well region serves as an isolation well, reducing parasitic capacitance.