A semiconductor device is formed by forming a deep trench in a substrate and a dielectric liner on sidewalls of the deep trench. A first undoped polysilicon layer is formed on the semiconductor device, extending into the deep trench on the dielectric liner, but not filling the deep trench. Dopants are implanted into the first polysilicon layer. A second layer of polysilicon is formed on the first layer of polysilicon. A thermal drive anneal activates and diffuses the dopants. In one version, the dielectric liner is removed at the bottom of the deep trench before the first polysilicon layer is formed, so that the polysilicon in the deep trench provides a contact to the substrate. In another version, the polysilicon in the deep trench is isolated from the substrate by the dielectric liner.