Methods of fabricating P-I-N diodes, structures for P-I-N diodes and design structure for P-I-N diodes. A method includes: forming a trench in a silicon substrate; forming a doped region in the substrate abutting the trench; growing an intrinsic epitaxial silicon layer on surfaces of the trench; depositing a doped polysilicon layer to fill remaining space in the trench, performing a chemical mechanical polish so top surfaces of the intrinsic epitaxial silicon layer and the doped polysilicon layer are coplanar; forming a dielectric isolation layer in the substrate; forming a dielectric layer on top of the isolation layer; and forming a first metal contact to the doped polysilicon layer through the dielectric layer and a second contact to the doped region the dielectric and through the isolation layer.