Semiconductor device structure and method for forming the same

A semiconductor device structure, comprising:

• a substrate having a first region and a second region;
• a plurality of first nanostructures stacked in a vertical direction in the first region;
• a plurality of second nanostructures stacked in the vertical direction in the second region;
• a semiconductor layer formed below the first nanostructures in the first region, wherein a material of the semiconductor layer is different from a material of the substrate;
• a first layer formed adjacent to the semiconductor layer;
• and
• a second layer formed adjacent to the second nanostructures, wherein a top surface of the first layer is higher than a top surface of the second layer.

A method for forming a semiconductor device structure, comprising:

• forming a first stack structure over a substrate, wherein the first stack structure comprises a plurality of first semiconductor layers and a plurality of second semiconductor layers stacked in a vertical direction;
• forming a dummy gate structure over the first stack structure;
• removing a portion of the first stack structure to form a first S/D trench;
• removing a portion of the first semiconductor layers to form recesses, wherein a bottommost first semiconductor layer remains;
• forming a sacrificial layer in the recesses, wherein the sacrificial layer is higher than a top surface of the bottommost first semiconductor layer;
• forming a first layer in the first S/D trench, wherein a top surface of the first layer is higher than a top surface of the bottommost first semiconductor layer;
• and
• forming a first source/drain (S/D) structure over the first layer.