Capacitance reduction for backside power rail device

A semiconductor device, comprising:

• first and second source/drain (S/D) structures in contact with opposite ends of a stack of channel structures;
• a first contact structure in contact with the first S/D structure;
• a dielectric structure on a first side of the second S/D structure;
• an air gap between the dielectric structure and the second S/D structure;
• and
• a second contact structure in contact with a second side of the second S/D structure, wherein the second side is opposite to the first side, and wherein the second S/D structure is between the air gap and the second contact structure.

A semiconductor device, comprising:

• first and second stacks of channel structures;
• a source/drain (S/D) structure between the first and second stacks of channel structures;
• a dielectric structure on a first side of the second S/D structure;
• an air gap between the dielectric structure and the S/D structure, wherein the air gap is between the first and second stacks of channel structures;
• and
• a contact structure in contact with a second side of the S/D structure, wherein the second side is opposite to the first side, and wherein the S/D structure is between the air gap and the second contact structure.