A semiconductor transistor device includes a channel structure, a gate structure, a first source/drain epitaxial structure, a second source/drain epitaxial structure, a gate contact, and a back-side source/drain contact. The gate structure wraps around the channel structure. The first source/drain epitaxial structure and the second source/drain epitaxial structure are disposed on opposite endings of the channel structure. The gate contact is disposed on the gate structure. The back-side source/drain contact is disposed under the first source/drain epitaxial structure. The first source/drain epitaxial structure has a concave bottom surface contacting the back-side source/drain contact.