Semiconductor device and manufacturing method thereof

A method comprising:

• forming a semiconductor structure over a substrate;
• forming an isolation structure over the substrate and adjacent the semiconductor structure;
• patterning the isolation structure to form an opening in the isolation structure and expose the substrate;
• filling conductive materials in the opening to form a conductive via in the isolation structure;
• after filling the conductive materials, forming a transistor over the substrate and the isolation structure, wherein a gate structure of the transistor is electrically connected to conductive via;
• removing the substrate to expose the gate structure;
• depositing a dielectric layer under the gate structure and the isolation structure;
• and
• forming a backside gate rail in the dielectric layer and connected to the conductive via.

A method comprising:

• forming a first transistor and a second transistor over a substrate, wherein the first transistor and the second transistor are surrounded by a interlayer dielectric (ILD) layer over the substrate, the first transistor and the second transistor are arranged in a direction in a top view, and a gate structure of the first transistor and a gate structure of the second transistor extend in the direction in the top view;
• forming a through via in the ILD layer and in contact with the substrate;
• forming a front-side gate rail over the first transistor and the second transistor and electrically connected to the gate structure of the first transistor, the gate structure of the second transistor, and the through via;
• removing the substrate to expose the through via;
• and
• forming a backside gate rail under the first transistor and the second transistor and electrically connected to the through via after removing the substrate.