Gate-all-around structure with self substrate isolation and methods of forming the same

A semiconductor device, comprising:

• a fin structure having a first doped layer disposed over a second doped layer, wherein the first doped layer has a first dopant concentration and the second doped layer has a second dopant concentration, wherein the first dopant concentration is greater than the second dopant concentration;
• a high-k dielectric layer disposed in the fin structure between the first doped layer and the second doped layer;
• a first source/drain feature and a second source/drain feature disposed on the first doped layer;
• a semiconductor layer disposed over the first doped layer, wherein the semiconductor layer is further disposed between the first source/drain feature and the second source/drain feature;
• and
• a gate stack surrounding the semiconductor layer, such that a portion of the gate stack is disposed between the first doped layer and the semiconductor layer.

A device comprising:

• a semiconductor fin structure disposed on a substrate, the semiconductor fin structure having a first dopant concentration;
• a dielectric layer disposed directly on a top surface of the semiconductor fin structure;
• an anti-punch through (APT) layer disposed over the dielectric layer, wherein the APT layer has a second dopant concentration that is greater than the first dopant concentration;
• an elongated semiconductor member disposed over the APT layer;
• a gate dielectric layer wrapping around the elongated semiconductor member;
• a gate electrode layer wrapping around the gate dielectric layer;
• and
• a first epitaxial source/drain feature disposed directly on the APT layer.