Transistor structures with reduced source/drain leakage through backside treatment of subfin semiconductor material

An integrated circuit (IC) structure, comprising:

• one or more levels of metallization on a frontside of a transistor structure, wherein the transistor structure further comprises:
• a channel portion of a fin;
• a source material and a drain material at opposite ends of channel portion, wherein the source and drain materials are crystalline;
• and
• a gate stack adjacent to the channel portion;
• and
• a base portion of the fin on a backside of the transistor structure, wherein:
• the gate stack is between, and in direct contact with, a center length of the base portion and the channel portion of the fin;
• a first end length of the base portion is in direct contact with the source material;
• a second end length of the base portion is in direct contact with the drain material;
• and
• the base portion is an amorphous material comprising oxygen, and Si or Ge and a metal impurity associated with a catalytic oxidation of the Si or Ge within the base portion;
• and
• an isolation dielectric adjacent to a sidewall of the base portion.