What is claimed is:1. A semiconductor device, comprising:a substrate having isolation structures therein; anda capacitor structure located on a top surface of the isolation structure, wherein the capacitor structure comprises a semiconductor material pattern and an insulator pattern inlaid in the semiconductor material pattern, wherein the semiconductor material pattern and the insulator pattern are located at a same horizontal level on the isolation structures;wherein the semiconductor material pattern comprises a lightly doped portion and a first heavily doped portion sandwiching the insulator pattern, and wherein the lightly doped portion has a doping concentration lower than that of the first heavily doped portion.2. The semiconductor device of claim 1, wherein the semiconductor material pattern further comprises a second heavily doped portion, contacting the lightly doped portion on opposite sides of the insulator pattern.3. The semiconductor device of claim 2, wherein the first and second heavily doped portions have a same doping concentration.4. The semiconductor device of claim 3, wherein the second heavily doped portion is a strip structure covering one sidewall surface of the lightly doped portion.5. The semiconductor device of claim 4, wherein the first and second heavily doped portions and the lightly doped portion are strip structures arranged in parallel, the insulator pattern is in contact with the first heavily doped portion and the lightly doped portion, and the lightly doped portion is in contact with the second heavily doped portion and the insulator pattern.6. The semiconductor device of claim 2, wherein the lightly doped portion has a doping concentration lower than that of the first and second heavily doped portions.7. The semiconductor device of claim 6, wherein the lightly doped portion includes a ring structure, the first heavily doped portion is located within the ring structure and is isolated from the lightly doped portion by the insulator pattern, and the second heavily doped portion comprises strip structures arranged at opposite sides the lightly doped portion and are in contact with the lightly doped portion.8. A semiconductor device, comprising:a substrate having isolation structures therein; anda capacitor structure located on a top surface of the isolation structure, wherein the capacitor structure comprises an insulator pattern with a ring shape and a semiconductor material pattern having a first heavily doped portion inside the insulator pattern and a second ring-shaped portion enclosing the insulator pattern, wherein the semiconductor material pattern and the insulator pattern are in contact with the top surface of the isolation structure.9. The semiconductor device of claim 8, wherein the second ring-shaped portion comprises a second heavily doped portion disposed on opposite sides of the insulator pattern.10. The semiconductor device of claim 9, wherein the first and second heavily doped portions are of a same conductive type and have a same doping concentration.11. The semiconductor device of claim 9, wherein the second ring-shaped portion further comprises a lightly doped portion located between the insulator pattern and the second heavily doped portion, wherein the insulator pattern is sandwiched between the first heavily doped portion and the lightly doped portion, and the lightly doped portion has a doping concentration lower than that of the first and second heavily doped portions.12. The semiconductor device of claim 11, wherein the first heavily doped portion is of a conductive type different from that of the lightly doped portion and the second heavily doped portion.13. The semiconductor device of claim 11, wherein the first and second heavily doped portions are strip structures arranged in parallel.14. The semiconductor device of claim 11, wherein the lightly doped portion has a ring shape.15. The semiconductor device of claim 14, wherein the second heavily doped portion comprises strip structures covering opposite sides of the lightly doped portion.16. The semiconductor device of claim 15, wherein the second heavily doped portion is in contact with the lightly doped portion.17. A semiconductor device, comprising:an isolation structure disposed in a substrate;a lightly doped semiconductor ring with a first doping concentration disposed on the isolation structure;a heavily doped semiconductor structure disposed in a center region of the lightly doped semiconductor ring and on the isolation structure, the heavily doped semiconductor structure has a second doping concentration higher than the first doping concentration; andan insulator material layer filling spaces between the lightly doped semiconductor ring and the heavily doped semiconductor structure.18. The semiconductor device of claim 17, further comprising a transistor disposed in an active region of the substrate, wherein the transistor has a lightly doped semiconductor region with the first doping concentration.19. The semiconductor device of claim 18, wherein the transistor has source and drain regions with the first doping concentration.20. The semiconductor device of claim 17, wherein the insulator material layer extends upwardly and covers top surfaces of the lightly doped semiconductor ring and the heavily doped semiconductor structure.