FIG. 11 is a schematic perspective view showing a portion of the structure including a capacitor structure in accordance with some embodiments of the disclosure. The structure of FIG. 11 may be fabricated following the process steps depicted from FIG. 1 to FIG. 5 and FIG. 7. Only a portion of the structure in the isolating region IR is shown in FIG. 11. As shown in FIG. 11, the structure includes spacers 150 formed on sidewalls of the portions 135B, 136A and 136B, while the locations for the to-be formed contacts are labelled CT. In FIG. 11, the lightly doped portions 136A form a ring-shaped structure, and the heavily doped portion 135B is located in the middle of the ring-shaped structure like an islet and spaced apart from the ring-shaped structure. In FIG. 11, the ILD layer 180 is formed between the lightly doped portions 136A and the heavily doped portions 136B. The ILD layer 180 is also formed outside the ring-shaped structure of the lightly doped portions 136A and the heavily doped portions 136B.