A high-power 320 GHz transmitter using 130 nm SiGe BiCMOS technology (f _T /f _max = 220/280 GHz) is reported. This transmitter consists of a 4 × 4 array of radiators based on coupled harmonic oscillators. By incorporating a signal filter structure called return-path gap coupler into a differential self-feeding oscillator, the proposed 320 GHz radiator simultaneously maximizes the fundamental oscillation power, harmonic generation, as well as on-chip radiation. To facilitate the TX-RX synchronization of a future terahertz (THz) heterodyne imaging chipset, a fully-integrated phase-locked loop (PLL) is also implemented in the transmitter. Such on-chip phase-locking capability is the first demonstration for all THz radiators in silicon. In the far-field measurement, the total radiated power and EIRP of the chip is 3.3 mW and 22.5 dBm, respectively. The transmitter consumes 610 mW DC power, which leads to a DC-to-THz radiation efficiency of 0.54%. To the authors' best knowledge, this work presents the highest radiated power and DC-to-THz radiation efficiency in silicon-based THz radiating sources.