This article presents a 135–155-GHz low-power and high-efficiency frequency multiply-by-9 (x9) frequency-modulated continuous-wave (FMCW) radar transmitter (TX). Starting from a 16-GHz frequency-chirping input signal, cascaded frequency triplers at $V$ -band (40–75 GHz) and $D$ -band (110–170 GHz) bring the signal to the $D$ -band, subsequently amplified and radiated via a power amplifier (PA) and an on-chip antenna at $D$ -band. The $D$ -band PA with a reduced gain of transistors at $f_{\max }$ /2 proposes a broadband gain-boosting technique, achieving a maximum achievable gain ( $G_{\max }$ ) for broad frequency range with high-order embedding passives. The x9 chain proposes phase-controlled frequency triplers that align the phase of each harmonic contribution and boost the third harmonic output power, conversion gain, and dc-to-RF efficiency. Implemented in a 28-nm CMOS process, the TX achieves a measured effective isotropic radiated power (EIRP) of 9.4 dBm, a dc-EIRP efficiency of 16.6% while exhibiting an antenna gain de-embedded output power of 7.1 dBm, and a dc-to-RF efficiency of 9.7% with less than 77-mW dc power consumption.