Structures for a vertical varactor diode and related methods

A method of forming a structure for a varactor diode, the method comprising:

• forming a first semiconductor layer including a first section, a second section, and a third section connecting the first section to the second section, wherein the third section is positioned on a substrate of a semiconductor-on-insulator substrate that includes a dielectric layer having a first interface with the substrate, the third section is thinner than the first section or the second section, the substrate comprises a semiconductor material, the first semiconductor layer has a first conductivity type, and the third section of the first semiconductor layer is surrounded by the dielectric layer;
• forming shallow trench isolation region extending around the first section of the first semiconductor layer;
• forming a second semiconductor layer on the section of the first semiconductor layer, wherein the second semiconductor layer comprises silicon-germanium having a second conductivity type opposite to the first conductivity type, and the second semiconductor layer abuts the section of the first semiconductor layer along a second interface;
• forming a third semiconductor layer on the second semiconductor layer, wherein the third semiconductor layer has the second conductivity type;
• forming a first doped region in the section of the first semiconductor layer, wherein the first doped region has the first conductivity type, the first doped region contains a higher concentration of a dopant of the first conductivity type than the section of the first semiconductor layer, and the first doped region is positioned adjacent to the second interface;
• and
• forming second doped region in the second section of the first semiconductor layer, wherein the second doped region has the first conductivity type.