Field effect transistor with reduced source/drain resistance

A semiconductor structure comprising:

• a semiconductor substrate;
• a semiconductor fin, protruding from the substrate;
• a top semiconductor layer at an upper end of the fin away from the substrate;
• a bottom semiconductor layer in the substrate adjacent to and partly underlying the fin;
• a gate stack surrounding vertical sides of the fin between the top semiconductor layer and the bottom semiconductor layer;
• an interlayer dielectric surrounding the fin and the gate stack;
• a metallic top contact penetrating the interlayer dielectric and touching the top semiconductor layer;
• a metallic gate contact penetrating the interlayer dielectric and touching the gate stack;
• a bypass layer partly underlying the fin and touching the bottom semiconductor layer;
• and
• a metallic bottom contact penetrating the interlayer dielectric and touching the bypass layer,
• wherein the bypass layer comprises a metastable alloy of two or more elements of semiconductors and dopants.

A method comprising:

• forming an asymmetric semiconductor structure that comprises:
• a fin that has an upper portion and a base;
• a top source/drain structure that touches the upper portion of the fin;
• a bottom source/drain structure that touches the base of the fin, wherein the top source/drain structure is smaller than the bottom source/drain structure;
• and
• a germanium-containing bypass layer within the bottom source/drain structure;
• alloying the germanium-containing bypass layer with a dopant to form a metastable semiconductor-dopant alloy;
• forming a top metallic contact that touches the top source/drain structure at a first conductive path length from the fin,
• forming a bottom metallic contact that touches the bypass layer at a second conductive path length from the fin;
• and
• wherein the second conductive path length is significantly longer than the first conductive path length, but the bypass layer is effective to reduce resistance along the second conductive path length so that a resistance from the bottom metallic contact to the fin is nearly equal to a resistance from the top metallic contact to the fin.