We report a circular-shape monolithic transistor-antenna (trantenna) for high-performance plasmonic terahertz (THz) detector. By designing an asymmetric transistor on a ring-type metal-gate structure, more enhanced (45 times) channel charge asymmetry has been obtained in comparison with a bar-type asymmetric transistor of our previous work. In addition, by exploiting ring-type transistor itself as a monolithic circular patch antenna, which is designed for a 0.12-THz resonance frequency, we demonstrated the highly-enhanced responsivity (R V ) > 1 kV/W (× 5) and reduced noise-equivalent power (NEP) < 10 pW/Hz 0.5 (× 1/10).