A compact monolithic trantenna ( <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tran</u> sistor-an <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tenna</u> ) device is presented for a high-performance sub-THz wave detector using 28-nm CMOS foundry process. Based on a highly localized plasmonic wave in a silicon nano-ring field-effect transistor (FET), we obtained a total 535-fold photoresponse ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta u$ </tex-math></inline-formula> ) enhancement in an on-chip measurement as compared with our previous works using the same asymmetry ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\eta }_a=30$ </tex-math></inline-formula> ). The inner contact diameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d_{\text{in}}$ </tex-math></inline-formula> ) was scaled down from 8 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.13\mu \text{m}$ </tex-math></inline-formula> for the parasitic resistance limit case. By changing the ground source from inside to outside the nano-ring FET, we could generate different <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta u$ </tex-math></inline-formula> polarities, which in turn reduced the junction leakage with improved <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|\Delta u|$ </tex-math></inline-formula> . From a fabricated nano-ring FET with the outer ring grounded source, we observed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times$ </tex-math></inline-formula> of additional <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|\Delta u|$ </tex-math></inline-formula> enhancement followed by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$107\times$ </tex-math></inline-formula> with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d_{\text{in}}$ </tex-math></inline-formula> scaling. In addition, based on the highly localized plasmonic wave nano-ring FET without any external gain, a record-high free-space responsivity of 12.4 kV/W and a reduced noise equivalent power of 1 pW/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}^{0.5}$ </tex-math></inline-formula> were experimentally demonstrated under 0.12-THz radiation.