A low-power and high efficiency 135-155 GHz FMCW radar transmitter (TX) in 28 nm CMOS is presented. Starting from a 16 GHz signal, the x9 frequency multiplier chain proposes phase-controlled frequency triplers at D-band and V-band exploiting phase aligned harmonic generation for high efficiency, conversion gain and output power, avoiding the use of power-hungry buffer stages. The PA proposes a wideband maximum achievable gain (Gmax) core to mitigate the gain limitation of D-band amplifiers. The design integrates an on-chip antenna and an optimized PA to antenna interface. The TX achieves a EIRP of 9.4 dBm with 9.7%/16.6% DC-RF/DC-EIRP efficiency which is the highest among the state-of-the-art D-band frequency multiplier chains multiplying more than four.