Capacitance reduction for back-side power rail device

A semiconductor transistor device, comprising:

• a channel structure;
• a gate structure underlying the channel structure;
• a first source/drain structure and a second source/drain structure disposed on opposite endings of the channel structure;
• a back-side source/drain contact disposed under and contacting the first source/drain structure;
• and
• a back-side dielectric cap disposed under the gate structure.

A method of forming a semiconductor transistor device, the method comprising:

• forming a fin structure over a substrate by alternately stacking first semiconductor layers and second semiconductor layers;
• forming a gate structure to cover a first portion of the fin structure and to leave a second portion of the fin structure uncovered;
• removing the second portion of the fin structure;
• forming a first source/drain epitaxial structure and a second source/drain epitaxial structure on opposite endings of the fin structure which corresponded to the second portion of the fin structure;
• removing the substrate to expose the first source/drain epitaxial structure or the second source/drain epitaxial structure;
• and
• forming a back-side source/drain contact under and contacting the first source/drain epitaxial structure or the second source/drain epitaxial structure.