Semiconductor device structure

A semiconductor device structure, comprising:

• a substrate having a base;
• a multilayer stack over the base, wherein the multilayer stack comprises a first channel layer and a second channel layer over and spaced apart from the first channel layer, and the first channel layer is between the second channel layer and the base;
• a gate stack over the substrate, wherein the gate stack wraps around the multilayer stack, and the gate stack has a bottommost part connected between the base and the first channel layer;
• an insulating feature over the base, wherein the insulating feature has a first top surface lower than a second top surface of the bottommost part and higher than a first bottom surface of the bottommost part;
• and
• a source/drain structure over the insulating feature and connected to the second channel layer, wherein the source/drain structure is electrically insulated from the first channel layer.

A semiconductor device structure, comprising:

• a substrate;
• a multilayer stack over the substrate, wherein the multilayer stack comprises a first channel layer and a second channel layer over and spaced apart from the first channel layer;
• a gate stack over the substrate, wherein the gate stack wraps around the multilayer stack;
• a source/drain structure over the substrate and connected to the second channel layer, wherein the source/drain structure is electrically insulated from the first channel layer;
• and
• an insulating feature between the substrate and the source/drain structure, wherein the insulating feature has a curved bottom surface facing the substrate.