A structure and method for fabricating a III-V compound semiconductor-containing heterostructure field-effect transistor (FET) with self-aligned and overlapped extensions using a replacement gate process is disclosed. The a III-V compound semiconductor-containing heterostructure field-effect transistor (FET) structure may be formed by forming a III-V compound semiconductor-containing heterostructure having multiple layers and a T-shaped gate structure using a gate replacement process. The T-shaped gate structure may be formed with a bottom surface substantially below an upper surface of the III-V compound semiconductor-containing heterostructure and an upper surface above the III-V compound semiconductor-containing heterostructure. An undoped region may be formed below the bottom surface of the T-shaped gate structure on a layer of the III-V compound semiconductor-containing heterostructure.